"Incident" is written in a ballad meter, and you can check out "Forms and Meter" for the deets on the form. But now we're concerned with how this baby sounds when you read it out loud.
And how does it sound? Jaunty and fun, we think. To use the poem's words, we think that the ballad form is inherently "Heart-filled, head-filled with glee" (2). Hear that jauntiness? Well, in the poetry biz that's called alliteration. The repetition of the H sounds at the front of "Heart-filled" and "head-filled" give off a playful vibe, as does the alliteration later in line 6 ("was" and "whit"). The effect is an upbeat form that sounds almost sing-songy. One of the great things about poems written in ballad meter is that you can actually sing them to the tune of the theme song for Gilligan's Island. If that ain't jaunty, we don't know what is.
But there's a real disconnect between what actually happens in the poem (a moment of revolting, racist speech) and its jaunty and light-hearted tone. This clash between form and content makes us all the more aware of the horribleness of the moment, because we, like the speaker, started off the poem in a ballad meter, with a happy-go-lucky feeling. The sounds in "Incident" play a big part in putting that initial spring in our step.
Finally, to experience this clash for yourself, check out this awesome rendition of the poem.