• for 16 lines before the first period finally appears. Although the structure of this sentence is not especially difficult to follow, neither can that structure be called easy or instantly accessible. Milton’s sentences often flow on and on, adding a kind of majesty to the phrasing of the work.
• A tendency to allude, either overtly or indirectly, to the Bible and to classical literature. Since the Christian Bible and the Greek and Roman classics were among the most important texts of Milton’s culture, his habitual allusions to them instantly elevate the tone of his phrasing.
• A tendency to use a kind of sentence structure more common in Latin than in simple Anglo-Saxon English.  For instance, in the first sentence of Paradise Lost, the very first verb does not appear until line 6. Instead of beginning the poem by writing, “Sing, Heav’nly Muse, of man’s first disobedience,” Milton does not provide the crucial verb “Sing” until line six.  This postponement of the verb creates a kind of suspense, as we wonder what verb will make sense of the first five lines. Another superb example of Milton’s use of “Latinate” syntax, or sentence structure, occurs, in lines 44-45, where Milton, speaking of Satan says,

Him the Almighty Power
Hurled headlong flaming from th’ethereal sky.

An example of the “grand style” at the beginning of Book 3 involves Milton’s reference to Hell as the “Stygian pool” (14). Rather than simply referring to “Hell” or even to “Hades,” Milton offers a classical allusion to enhance the grandeur and resonance of his language. Another classical allusion appears two lines later, in the reference to “th’ Orphéan lyre” (17). This tendency to discuss Christian ideas by alluding to classical precedents is typical of the entire poem. Milton was steeped in the Bible, but he was also thoroughly familiar with the Greek and Latin classics and used his classical learning to enhance the richness of his style. Thus in one line he refers to the classical “Muses” (27), and then three lines later he refers to Biblical “Sion” (30). This constant interplay of the classical and the Christian is one of the main features of Milton’s “grand style.”

For an excellent brief overview of the poem, please see C. S. Lewis, A Preface to Paradise Lost (Oxford: Oxford University Press, 1961).

Milton had soaring ambitions for Paradise Lost. He hoped to create an epic that would stand alongside the works of Homer, Vergil, and Dante, and that would be unparalleled in the English language. The theme he chose is a lofty one, and the language that he used is tailored to the subject matter, as he demonstrates in the beginning of Book One:

Sing Heav'nly Muse, that on the secret top
Of Oreb, or of Sinai, didst inspire
That Shepherd, who first taught the chosen Seed,
In the Beginning how the Heav'ns and Earth
Rose out of Chaos...I thence
Invoke thy aid to my adventurous Song;
That with no middle flight intends to soar
Above th' Aonian Mount while it pursues 
Things unattempted yet in Prose or Rhime.

This excerpt is a deliberate nod to Homer, who also included evocations of Muses. It also expresses his ambition to write of "Things unattempted," i.e., to "justifye the wayes of God to men."  He continues to employ grand, sweeping language as he describes the titanic struggle between God and Lucifer, one which left the rebellious angel cast down 

With hideous ruine and combustion down
To bottomless perdition, there to dwell
In Adamantine Chains and penal Fire,
Who durst defie th' Omnipotent to Arms.

It is, perhaps, precisely this majestic language that makes Paradise Lost such a challenge to modern readers, but it is also suited to the epic themes of combat between God and Lucifer, one in which man is swept up. It is fundamentally a struggle between good and evil, and Milton wants to show both the majesty of the former and the power of the latter. His success in doing so, and in reimagining a foundational story in Christian tradition in epic terms, owes much to his elevated style.

The best word to describe Milton's style in Paradise Lost is “grand.” Paradise Lost is epic poetry, and Milton elevates his style to match the seriousness and grandeur of his subject matter. Think, for instance, of this famous quotation:

The mind is its own place, and in it self
Can make a Heav'n of Hell, a Hell of Heav'n.

Notice the high tone here, the seriousness, the forcefulness of the assertion, and the language. This is a grand claim for a grand poem.

Scholars also call Milton's style Latinate. Milton often imitates the word order of Latin rather than sticking with English syntax, and Latin word order is much freer than English word order (Latin being an inflected language with word endings that show the function of a word in a sentence). This makes his poetry dense and sometimes quite difficult to follow, but it also increases readers' attention (as they struggle to figure out Milton's sentences) and adds another level of dignity and seriousness to the lines. Look at these lines, for example:

This inaccessible high strength, the seat
of deity supreme, us dispossessed,
He trusted to have seized.

The object (seat) appears first surrounded by modifiers. Milton includes what in Latin would be ablative absolute (“us dispossessed”) but in English throws in yet another phrase to add further information. The subject (he) doesn't appear until the third line. This is very much Latin in style, and it is difficult to parse in English.

Further, Milton's style is highly allusive. He throws all kinds of biblical and classical references into Paradise Lost, expecting that his readers will know what he means. Milton was an extremely learned man; these references were part of his repertoire, part of his intellectual landscape. But they certainly give modern readers plenty to look up. Once, for instance, Milton writes, “Whose waves overthrow / Busiris and his Memphian chivalry.” He is talking about the Egyptians drowned in the Red Sea after the Israelites passed through, but his allusiveness makes this unclear until we discover that Busiris is the mythical son of Egypt and that Memphis, in this case, is a city in Egypt.

Milton also creatively uses extremely high-level vocabulary, again elevating his style to the heights of learned grandeur. Phrases like “the palpable obscure” and “wanted calm” make readers sit up and take notice (and also reach for their dictionaries). Again, Milton's subject matter is high, and he intends for his expression to match it, reaching to the heights and depths into which he carries his readers.

This is a huge question so I will focus on some of the important characteristics of Milton's style. First, yes, it is an epic poem, and Milton follows the traditional epic form, giving the reader war and love, the supernatural, the descent into hell, a catalogue of warriors. Yet that tells us little about its style. Milton's style is dense, Baroque, packed with similes and metaphors, complex sentences, and overflowing with rich descriptions and images. It is the opposite of stark. It is the opposite of modest. Instead, Milton struts his stuff, including his skill with language, his unabashed exuberance and sense of the importance of his subject, and his immense knowledge.

The poem is extraordinarily allusive. That means it references a vast number of other literary works. Milton was enormously well read and he weaves his knowledge into the rich stylistic tapestry of this grand work. 

Let's look at just one passage, in book 1, lines 65-83:

Regions of sorrow, doleful shades, where peace [65]

And rest can never dwell, hope never comes

That comes to all; but torture without end
Still urges, and a fiery Deluge, fed
With ever-burning Sulphur unconsum'd:
Such place Eternal Justice had prepar'd [70]
For those rebellious, here thir Prison ordain'd
In utter darkness, and thir portion set
As far remov'd from God and light of Heav'n
As from the Center thrice to th' utmost Pole.
O how unlike the place from whence they fell! [75]
There the companions of his fall, o'rewhelm'd
With Floods and Whirlwinds of tempestuous fire,
He soon discerns, and weltring by his side
One next himself in power, and next in crime,
Long after known in Palestine, and nam'd [80]
Beelzebub. To whom th' Arch-Enemy,
And thence in Heav'n call'd Satan, with bold words
Breaking the horrid silence thus began.

Milton describes hell, but he doesn't just say it is a place of fire and darkness: he gives an extended, richly imagined and unrestrained description of it: "a fiery Deluge," "ever burning sulphur unconsumed," "utter darkness," "floods and whirlwinds of tempestuous fire." This is an active, writhing place, full of motion, similar to a Baroque painting. The passage is also full of allusions. For example, "from the center thrice" alludes to the Ptolemaic cosmos, while Beelzebub is a deity whose name means "lord of the flies." Milton speaks in a voice of utter confidence as he introduces characters and, most importantly, grandly sets the stage for Satan's speech. 

A primary feature of John Milton's style in Paradise Lost in particular is his use of enjambment: verses (lines) that have no end punctuation but are rolled over to the next verse that will contain punctuation, "Yet chains in Hell, not realms, expect: Mean while /  From me returned,..." Due to enjambment, Milton's punctuation may fall within the verse or at the end, "Thyself not free, but to thyself enthralled; /." Another interesting feature of Milton's punctuation is the frequency with which he uses the colon (:). Colons in poetry are not unknown, Goethe uses a well-placed colon here and there, but Milton gives the colon a place of punctuational honor that is uncommon.

Milton's style is first of all epic; he is consciously writing an epic poem, modeling it after the great epics of the classical past.

His language used is Latinate; he considered writing the poem in Latin, and adapts many terms from Latin. This makes his style seem more formal, and makes many terms more complex than if drawing on more familiar English words.

It is written in blank verse, and iambic pentameter.

He uses many allusions to classical and exotic topics, adding weight and grandeur to the style.

The so-called “grand” or lofty style of Milton’s Paradise Lost is significant to the poem in numerous ways, chiefly because it struck Milton as by far the style most appropriate to the lofty subject matter he had chosen for his poem. Rather than choosing a “low” (that is, a common or colloquial) style or the kind of “middle” style used in much conventional poetry, Milton chose an elevated style because it seemed the only style that could do justice to the important matters he meant to discuss and depict.

Several elements of Milton’s “grand style” might be listed as follows:

• a tendency to use long and complex sentences
• a tendency to employ learned allusions, especially to the Bible and to the Greek and Roman classics
• a relative absence of obvious humor, especially any crude humor
• a tendency to choose unusual words in place of simple, common words
• a tendency to construct sentences in ways that resemble sentence structures often found in Latin
• a general (but not total) tendency to avoid crudeness of any kind, especially sexual crudity
• a tendency to display the learning of the poet and to expect similar learning in the poem’s readers

Many aspects of this “grand style” that illustrate its significance can be seen at the very beginning of Book 3, when Milton invokes divine inspiration by addressing a hymn to “holy Light” (1). Here Light is personified, so that it seems more than a mere physical fact but instead seems something living, even divine, and thus deserving of the dignity of the so-called “grand style.” Another example of the “grand style” in this passage appears in line 6, when Milton uses numerous words of Latin origin to describe Light as a “Bright effluence of bright essence increate” (6). This is not simple, plain, unadorned, Anglo-Saxon phrasing; rather, it is the kind of lofty phrasing Milton considered appropriate to his highly important topics. Milton can write very simple English, as when he refers to “The rising world of waters dark and deep” (3.11), but even here there is a touch of Latin sentence structure, since the adjectives follow the noun rather than preceding it.

A different kind of writer, with different purposes, would have written, “The Almighty power hurled him . . . .” Milton, however, makes his phrasing sound like a grand or elevated kind of English by using this kind of “Latinate” sentence structure.

A tendency to make learned allusions, as in the reference to “th’Aonian mount” in line 15.  Instead of referring openly to “Helicon,” a name with which many more readers would have been familiar, Milton chooses to refer to “th’ Aonian mount,” partly to display his own learning, partly to test, challenge, and stimulate the learning of his readers, and partly to make the phrasing unusual or above the ordinary.

For more on this topic, see Christopher Ricks, Milton’s Grand Style (Oxford: Oxford University Press, 1963).