Diction means choice of words.  The diction in Ozymandias is lofty, slightly unusual, and it paints vivid pictures.

By "lofty," I mean the author often chooses longer words that would be familiar to an educated person.  For example, the author uses "antique" instead of "ancient," "visage" instead of "face," and "colossal" instead of "huge."  This is formal, classroom diction, not the diction of the home or street.

Sometimes the poem uses a word that is not exactly loftier than the expected word, but it is a little different.  For example, the fallen statue is called a "wreck" instead of a "ruin" as we would expect.  Wreck is more often applied to ships, not ancient stone statues.

Sometimes these slightly odd word choices help the rhythm or rhyme of the poem.  For example, "Nothing beside remains." If the poet had written, "Nothing else remains," that line would not have had the right number of syllables.  "The lone and level sands stretch far away" sounds better than "The lonely, level sands stretch far away."  Stone in a desert does not really decay, but "Round the decay" rhymes with "far away" later, and matches it thematically as well. 

Finally, sometimes the author's diction allows him to paint a vivid picture more quickly than with more ordinary words.  "Two vast and trunkless legs of stone" is more concise than saying "The two remaining legs of a huge statue, the rest of which has fallen down," and it is more vivid too, since it emphasizes both the size and oddity of the legs.  "Those passions ... Which yet survive, stamped on these lifeless things" is more concise and more vivid than saying "The subject's attitudes, which are still clearly visible on the face of the statue."

"Ozymandias" by Percy Bysshe Shelley is an unusual sonnet.  It has fourteen lines; yet, in the three quatrains of the poem, the thought from each is carried over to the next quatrain.  The couplet is not two lines unto themselves nor do the couplet lines rhyme.  His rhyme scheme is atypical of the usual sonnet rhyming pattern.

The poem is told in first person. The narrator remembers meeting a man from an aged land who tells a story. The traveler came upon the ruins of a gigantic statue. The legs were still standing. However, the face of this statue was partially down in the sand. It portrayed a man with a scowling frown. He obviously had been a great leader. The sculptor was able to see into the character of the leader and his passions. But the stones are lifeless.

The statue's pedestal had been engraved with a quote:

My name is Ozymandias, King of Kings:
Look on my works, ye mighty, and despair

The great king Ozymandias [an ancient name for Rameses  the II] wanted to be remembered for the great architecture and other things that he brought to Egypt. Sadly, the sand, wind, and time brought all of it down to ruins and much of it became buried beneath the sand.

When the traveler looks at the pedestal and sees the ironic phrase, it seems pathetic and almost humorous. The entire poem is a metaphor for the foolishness of a man who thinks that anyone can harness time.  Ozymandias boasted of his accomplishments, which now are nothing but fodder for the sand and the wind.

A mighty king had a statue labeled so that people would know that his kingdom was wondrous in its beauty. Now, it is in ruins with most of it completely gone. Left are the legs, part of the face, and a pedestal with an engraving addressing his fabulous accomplishments. If the pharoah, could only see what was left to show mankind, what would he think?

The poem has a couple of lines of alliteration:

Of that colossal wreck, boundless and bare, 
The lone and level sands stretch far away

which emphasize the loneliness and barren ruins out in the middle of nowhere.

The irony of the entire reference to Ozymandius becomes the center of the poem. The great Rameses II and his statue lay in ruins from the natural deterioration from sand and time.

Although "figures of speech" can be considered broadly to include both poetic (sound) techniques as well as non-literal language, this answer is restricted to figures of speech that are non-literal language.

There are two instances of synecdoche in the poem. With synecdoche, a writer uses a part of the thing to represent the whole. Thus "the hand that mocked them, and the heart that fed" uses two parts of the king, his hand and his heart, to represent him. 

The use of the word "antique" in line 1 could be considered hyperbole. Hyperbole is an exaggeration for effect. The word "antique" can mean "something belonging to ancient times." In literal terms, a land cannot belong to ancient times. If the land exists in the present, it "belongs" in the present, so to call a land "antique" is an exaggeration, but it conveys the idea that the land was more important in ancient days. 

The idea that the "shattered visage" is able to "tell" about the sculptor is personification, which imparts human characteristics to inanimate objects. The visage is unable to communicate with words, so it literally cannot "tell" anything. The word "tell" is also a pun, or a play on words. Although the statue cannot "tell" the viewer anything, the viewer can "tell" from the statue certain things about the king. This is a different meaning of "tell," meaning to determine, but is suggested by the way in which it is used here.

The word "decay" implies a metaphor. Although structures can be said to "decay" when they deteriorate, decay more often brings to mind the destruction of organic matter by bacteria and other creatures. Just as this structure has toppled, so the king's body has decayed and decomposed long ago. The broken statue is a metaphor, or comparison, to the dead king. 

The word "colossal" might be considered both an allusion and a tautology. Although the word means simply "extremely large," it is derived from "colossus," which means a giant statue and was first used by Herodotus to describe the statues in ancient Egyptian temples. Thus it can be an allusion, a reference to a historical or mythical person or event. It also calls to mind the Rhodes Colossus, one of the seven wonders of the ancient world that was destroyed in an earthquake. So to call the wrecked statue a "colossal Wreck" could be like saying it is a "wrecked statue Wreck," making it a tautology, a way of saying the same thing with different words.

Although not captured in any specific line, the entire poem has an air of understatement and irony. The fact that no judgment is stated about the described scene but that it is left to speak for itself is understatement. Understatement deliberately makes something less important than it is. This scene is obviously quite impressive and meaningful, yet the meaning is not overtly stated. Irony represents a turn of events that is the opposite of what is expected. In this poem, the powerful king inspired fear in his day, but now his "Works" are nowhere to be seen. 

These figures of speech, examples of non-literal language, in "Ozymandias" give it a very rich meaning.

The speaker who begins the poem relates a tale once told to him by a traveler he'd met. This traveler had been to the desert, where he saw the broken pieces of a once-great statue all scattered and half-buried by the sand. The traveler saw "trunkless legs" here and the bits of a shattered face there, as well as the pedestal on which the effigy used to stand. From the inscription on that pedestal, he learned that the statue depicted a man called Ozymandias, a king who evidently believed that the many great deeds he accomplished in life would grant him a kind of immortality and that his fame would never die. However, when the traveler describes the "colossal Wreck," he is referring to the wreck of the statue that commemorated Ozymandias, a statue that has all but been lost to the sands of time. From this, we can see that the statue is a symbol of human mortality; we are all forgotten eventually.

The two existing educator answers have already addressed the key things to be aware of in terms of metaphors in this poem. There are few "obvious" metaphors; we do not see the most common metaphorical construction in which something is stated to be something else without literal truth ("the curtain of darkness," "the moon's a balloon"). We can find some examples; "the heart that fed," for example, is a metaphor. The heart did not literally feed but rather it metaphorically sustained. The "hand that mocked," similarly, is a type of rhetorical device known as synecdoche, in which the "hand" does not mean literally the hand alone, but is a representation of the person who owned the hand and who did the "mocking."

In a broader sense, however, the "trunkless legs of stone" and the "shattered visage" which lies "half sunk" in the sand is a metaphorical representation of Ozymandias's hubris. The inscription on the pedestal has become ironic because Ozymandias once expected the mighty to "despair" because his works were so evident and vast, and yet now the "despair" comes only from the fact that the statue is a "Wreck" and "nothing beside remains" of Ozymandias's once great works. The statue, submerged in the desert, is a symbol of pride before a fall, Ozymandias's misplaced belief in his own longevity now sunk in the middle of nowhere, with no indication of what he had to be proud of.

A metaphor directly equates two things which in literal terms are unrelated. There are multiple examples of metaphor in "Ozymandias," not limited to the ones listed below.

The first is found in the first line, "a traveller from an antique land." Normally, we reserve the word "antique" for old objects, specifically ones which can be collected or easily owned. We would not normally apply the term "antique" to a region or country, so this association may be intended to convey something of the speaker's poor, stereotypical, or one-dimensional understanding of the traveller's origin; perhaps he sees the land as "antique" in the same way that popular generalizations of "the Orient" or "the jungles of Africa" do not really represent those places in truth.

Another metaphor is the "sneer of cold command." Command is not literally cold, as in temperature, but cold in terms of human emotion and empathy. The "shattered visage" is described as cruel, dispassionate, and haughty.

Shelley comments on the transitory nature of authority, power, prestige, and existence, while simultaneously examining how one's art can also have ephemeral qualities by portraying the dilapidated, neglected statue of Ozymandias lying in the middle of a barren desert. The once powerful king's statue is described as being a "colossal Wreck" and is nothing more than legs of stone and a half-buried shattered "visage." The colossal size of Ramses's statue symbolizes his lofty ambitions, inflated self-perception, and narcissistic personality, which is further emphasized by the inscription on the pedestal of the statue. Ramses's statue was originally constructed to portray his self-proclaimed omnipotence but erodes with time into a deteriorating, shattered structure. The decaying state of Ramses's statue also symbolically represents the transience of political leaders and regimes, and emphasizes the erosive processes of time.

I'm not exactly sure what you are asking about the poem by Shelley.  The poem involves a description of a statue that is now in ruins.  The speaker is a traveller from an "unknown land," and he is describing a once magnificent statue, probably that of Ramses II.  All that is left of the statue is the head and the feet.  On its pedestal reads the ironic quotation, "Look on these works, ye mighty, and despair!"  And of course, there are no works that have withstood time's erosion.  So, what then, does this statue symbolize?  It symbolizes perhaps man's yearning to make a permanent mark on the earth, and ultimately his inability to do so.  Eventually all our efforts are overcome by nature's forces.  The magnificent pharoah Ramses II's work has all vanished.  The sculptor's fashioning of the statue of this pharoah is now broken and in ruins.  Neither the work of a king nor the work of an artist can remain.  The forces of nature will overcome all.

Figurative language allows poets and writers to express concepts in unusual ways, making associations where ordinarily there would be no connection. Simile, metaphor, personification, alliteration, hyperbole and so on are all used to make descriptions and create images in the mind's eye. By using figures of speech, readers can get a visual picture of what the author or poet wants to express.  

In "Ozymandias," Shelley very quickly destroys any sense of grandeur surrounding the statue. The mocking tone confirms the irony in the words inscribed on the stone. Nothing lasts forever, not even the perceived sense of worth of "Ozymandias, King of Kings." By using figurative language, Shelley is able to create sharp contrast between what Ozymandias may have perceived would be the benefit of his statue and the reality of the situation, wherein his pride in his "works" reveals nothing more than a "colossal wreck." Hyperbole in the use of "colossal" allows the reader to imagine the sheer scale of destruction—far more than just a statue but a whole empire.

Shelley continues to explain the magnitude of the apparent loss of Ozymandias's kingdom by using alliteration, a sound device, evident in the words, "boundless and bare, The lone and level sands stretch..." This adds emphasis to the endlessness and uselessness of the situation in which the statue now lies and helps Shelley confirm his message. Shelley delivers a warning that no matter how "mighty" a person considers himself to be, the danger is that he will be reduced to nothing more than "trunkless legs of stone." How much better would it be to be remembered as a man of compassion rather than to be remembered for "a sneer of cold command." 

The primary element of figurative language in the poem is that of irony. All that remains of the great "works" of this "king of kings" is the remains of the monument Ozymandias had ordered to be carved in his own image, a reflection of his egotism. Ozymandias once taunted the "Mighty" to look upon his power and "despair." All that remains of his empire, however, is a decaying "colossal wreck," broken and half-buried in the "lone and level sands" that "stretch far away." The contrast between the arrogance of Ozymandias' words and the seemingly endless emptiness surrounding his "vast and trunkless legs of stone" emphasizes the poem's irony.

In the fifth line of Percy Bysshe Shelley's celebrated poem "Ozymandias," he employs the literary device known as personification. Shelly utilizes personification by writing, "And wrinkled lip, and sneer of cold command." Personification is a literary device in which the author applies human attributes and characteristics to animals or inanimate objects. Shelley utilizes personification in the fifth line by attributing the human characteristic of contempt to a lifeless statue. The statue is incapable of sneering or possessing a cold command because it is an inanimate object. Through personification, Shelly is emphasizing the contemptuous nature of Ramses II and highlighting his domineering personality. The scornful look on the statue’s face underscores the callous, disdainful personality of the former tyrant.

The frowning, sneering visage of Ramses II portrays him as a ruthless tyrant who aspired to intimidate his enemies and oppress his subjects. The traveler proceeds to describe Ramses II's merciless, cruel disposition and illustrates his tyrannical reign by describing the threatening message written on the pedestal of the statue. The intimidating, scornful appearance of the once-powerful ruler is juxtaposed to the statue's decaying, broken structure in the middle of the empty desert. Despite Ramses II's authority, time and nature have made a mockery of his grand statue and legacy.

Shelley's intention is to show that political power is never permanent and that even the greatest, most powerful ruler will not survive forever; thus Shelley seeks to satirize politicians, or dominant rulers, and call the reader's attention to the passing of political power. The poem has a weighty theme, so Shelley has used the formal sonnet form. 

The "colossal wreck" is literally the giant remains of the statue of the once-powerful ruler Ozymandias, with the "shattered visage" (face and expression) and the now broken "legs of stone" which have parted from the body so that they are "trunkless." The adjective "colossal" is also used ironically and metaphorically to refer to the enormous power this "king of kings" once had but has now lost, as Ozymandias has succumbed to death. Even his statue lies wrecked in the "lone and level" sands, which will endure far longer than he did! We notice the alliteration in this line, suggesting something long-lasting, even eternal. Therefore, the "sands" symbolize the impassive natural world, which is stronger than any political power or whim, which lasts for a comparatively brief time compared to the earth, which has witnessed man's changing ideas and histories for centuries. 

I would probably preference the idea of "the personal response" is something that is impossible to achieve.  There are many different and divergent personal responses to Shelley's poem.  I am sure that this can be seen in the responses that have been posted in this setting.  The metaphor of the broken and decrepit statue that once meant so much and stood so powerful is a wonderful statement on what it means to be human.  In contrast to the Neoclassicists, who believed that human achievement and human greatness would stand the test of time, I think that the Romantic thinkers believed in the idea that human beings' time is temporary and passing.  To make the life led as something meaningful and not waiting for immortality to make a judgment, to "seize the day" as opposed to standing in wait of it, is where the metaphor of the broken statue has a great deal of meaning.  In terms of the relationship between pride and a fall, I think that there is a great deal of truth to this as the statue which is meant to commemorate "the king of kings" is deserted, isolated, set apart from all else and is crumbling.  This helps to bring the idea that outside of pride, ruling and kingdom without meaningful accomplishment is not power in its own right nor is it representative of what strength is. The notion of pride helps to connect to the overall theme that anyone who rules in the hopes of being "king of kings" is going to suffer the fate of Ozymandias in the poem.

I like the question and the comments by the previous poster, but I want to pont out some small details in lines 4-8 of Shelley's short poem "Ozymandias" that may indeed support the theme (or moral, or even cliche) of "pride comes before the fall." The poem describes the head of the statue of the long dead king (or pharaoh) as follows: 

Half sunk, a shattered visage lies, whose frown
And wrinkled lip, and sneer of cold command
Tell that its sculptor well those passions read
Which yet survive, stamped on these lifeless things,
The hand that mocked them and the heart that fed.

The poem states that the facial expressions on the statue capture the essence of the man whom the statue represents, and these facial expressions are hardly ones of humility, compassion, or anything else that we might see as the opposite of pride. In fact, that "sneer of cold command" suggests to me that the ruler was just about as full of himself as he could be, and the very order that a huge statue be erected in his honor (a statue showing that very sneer) is a sign of his excessive pride.

To end my post in a summarizing statement, as did the previous poster, I think it's possible to say that the statue's head fell and shattered because it was so huge and so high up -- much like the man represented by the statue, who built himself up so much ("king of kings," indeed!) that he had nowhere to go but down. For the record, I don't normally like reading literature for moral lessons or reducing the meaning of literary works to what often sound to me like cliches, but in this case, such an approach has some merit.

For a metaphor, what about the "big head" in the poem. The literal big head is the shattered "visage" of the statue; as metaphor, it might refer as well to the king's excessively inflated sense of importance.

For alliteration, there are a few more that haven't been mentioned. One line has a repeated "h" sound -- "The hand that mocked them and the heart that fed" -- and later lines have other repeated consonants: "boundless and bare" and "low and level." Alliterating words don't have to be right next to each other.

I would not exactly say that the theme is pride goes before a fall.  To me, that implies that pride causes the "fall."  I think that the theme is more that everyone, no matter how important, becomes unimportant in the end.  Ozymandias did not cause his own downfall by being proud.  He "fell" simply because time passed.

There are a few examples of alliteration in this poem, lines like "cold command" and "survive, stamped."

To me, the whole poem is a metaphor.  I think the statue itself is a metaphor for fame and importance.