What's this poem sound like? A caffeinated rant, that's what! Though Lowell doesn't make use of conventional poetical sounds like rhyme, meter, or alliteration, he crams a ton into this poem, making for a more natural, energetic sound.
One thing to pay attention to is how he varies the length of his lines and sentences. For example, the first stanza is chock-full of adjectives, and if you're reading it aloud (go ahead, do it), you have to go pretty slowly so you don't stumble. But then he slows down to even things out with super-short sentences like, "My hand draws back."
Or later on, he has lines as short as two words ("Colonel Shaw"—line 61), following a previous long line ("the drained faces of Negro school-children rise like balloons"—line 60). This variation of length and pace makes for a somewhat natural-sounding poem (not unlike speech), so that the rather long poem seems to pass in a flash.