"The Lady of Shalott" uses aaabaaab rhyme to establish its dreamlike rhythm and structure, with the final word of each stanza almost always being "Shalott" (on one occasion, the word is "Lancelot"). This repetition alludes to the Matter of Britain tradition from which Tennyson draws his inspiration, a refrain of the sort frequently found in oral-tradition stories and folk tales.

There are many figurative and metaphorical devices within the poem. In the first stanza, we see an example of pathetic fallacy, where nature is described behaving in a way that supports the activity of the story: here, nature is personified and seemingly protecting Shalott, with the "long fields" there to "clothe the wold and meet the sky," and the waterlily and daffodil encircling Shalott. We see this later, where "the little isle is all in'railed." Like the Lady herself, Shalott is encircled, arguably either protected or isolated.

Repetition enforces the Lady's situation: we see the word "weaving" stated over and over to emphasize that this is her only task and consumes her every waking hour. By contrast, Sir Lancelot is compared figuratively to light and stars, signifying his difference from all others; Lancelot's horse's bridle "glitter'd . . . like to some branch of stars we see."

There are four stanzas in Parts I and II, five stanzas in Part III, and six in Part IV. The stanzas all contain the same basic structure: there are nine lines, with a rhyme scheme of aaaabcccb. This strong emphasis on rhymes helps to give the poem the feeling of an ancient tale.

What are the poetic devices (simile, metaphor, etc.) and their effects in "The Lady of Shalott"?

Tennyson's use of figurative language heightens the  power and meaning behind the words in the poem.  I have selected two examples of simile and metaphor in which the connotation of the brings deeper meaning to the characterization and development of the poem.

Simile--Compares unlike objects using 'like' or 'as'.  

The gemmy bridle glitter'd free,

Like to some branch of stars we see

Hung in the golden Galaxy.

Tennyson uses the a simile to compare the brilliance of Lancelot's ensemble to the stars; not only does he create dazzling imagery through this comparison, but the reader can also interpret that Lancelot shares other similarities to the "branch of stars" in that he is equally unattainable.

Metaphor-- Compares unlike objects without using 'like' or 'as'.

Tennyson compares the Lady's weaving to that of a "magic web," creating a spider metaphor.  Like his use of simile, Tennyson's use of metaphor also brings deeper meaning to the text.  The comparison of the Lady's textile to a web brings to mind how spiders use webs to trap their prey; however, in "The Lady of Shalott," the reverse seems true-- the Lady's web has entrapped her.  Only in Part III of the poem does the Lady break free from the web of her own making and flings it away, but just as the spider depends on the web for sustenance and survival, so does the Lady; abandoning her craft brings about her eventual doom.