The poem might praise laziness, but the style is anything but. Keats uses alliteration, caesura, anaphora, and internal rhyme to develop the poem's musical elements.
For example:
My sleep had been embroider'd with dim dreams;
My soul had been a lawn besprinkled o'er
With flowers, and stirring shades, and baffled beams: (42-44)
Notice the repetition of some similar sounds? "Sleep" and "soul," "dim" and "dreams," "stirring" and "shades," "baffled" and "beams"—all repeat initial consonant sounds. That's alliteration at work, a sonic device that Keats uses in this poem again and again.
Alliteration isn't the only trick Keats has in his bag, though. He's also using anaphora, the repetition of initial phrases at the beginning of clauses:
How is it, Shadows! that I knew ye not?
How came ye muffled in so hush a mask? (11-12)
The repetition of "How" in these lines ties them together, unlike another poetic device that we find in this poem: caesura. A caesura is a pause in the middle of a line, like the exclamation point after "Shadows!" Here's another example:
To steal away, and leave without a task
My idle days? Ripe was the drowsy hour; (14-15)
This time, the speaker uses a question mark to make the reader pause and take a breath. Give it a read aloud and you'll see what we mean.
And that's probably not the only thing you'll notice. "Away" and "days" rhyme, and because that rhyme occurs in the middle of lines as opposed to the end of them, it's called an internal rhyme. Even though there is caesura to interrupt these lines, then, the internal rhyme still brings them together in your mind's ear—er, if you can picture that.
So what's with all these techniques? Why does Keats put on a virtuoso display of sonic skills? One reason might be just the sheer irony of doing all this work in a poem called "Ode on Indolence." If he can pull all this off, how into naps is he really? All those devices make a poem about laziness sound anything but lazy.