This poem sounds a lot like a car shifting gears. The same high-quality motor is present the whole time, but it jumps between different levels of intensity. The poem begins in a low gear, with the slow thuds of d and u sounds: "drowsy," "numbness," "drunk," "drains," "sunk," "Dryad." The engine is just kind of idling, but you can tell that enormous power lies behind it.
Sure enough, in the second stanza, the poem takes on new intensity. as the speaker begins to express his bittersweet desire rather than just telling us about it. He speaks in exclamations like, "O for a beaker full of the warm South!" This down-and-up progression continues through the third and fourth stanzas. The third stanza slows down to focus on the word "where" that frames the human world, but this depressing lull only motivates him to shift to high gear in order to escape. The exclamations return in the fourth stanza, as he cries out for a change: "Away! Away!"
These shifts between manic and quiet moods are a hallmark of Keats's poetry, and the rest of the poem follows a similar pattern. The speaker slows down in the next few stanzas to smell the flowers (literally), and also to listen to the nightingale singing at night. The thought of his own death brings him back to cruising speed with a jolt, and he declares that, "Thou wast not born for death, immortal Bird!" (line 61). The poem ends in this higher gear, as the speaker's words follow behind the departing nightingale, crying out, "Adieu! Adieu!" like a brokenhearted lover watching his beloved flee. This poem may have a nice engine, but the nightingale's is even better.