Warning: I am answering this as a linguist, not a literary critic.
One of the eternal debates of the popular arts is whether rap music is a form of poetry. English teacher Christina M. Rau claims that it’s not in this guest commentary from The Irascible Professor.
My answer is that it is although a lot of rap is BAD POETRY, it is in fact poetry. That is, rappers are manipulating their lyrics to fit a metrical pattern. If it doesn’t fit the metrical pattern, it’s not rap. You can’t read any blank verse poem as is and have it be accepted as rap…it will just be blank verse poetry.
The problem is that mediocre rap tends to repeat the same phrase over and over and restrict their themes to “booty”. The good rap tends to include social commentary (some serious, most more light hearted) and have more complex lyrics. Here’s an example of what I would call a good rap song (from Eminem).
The traditional line break looks like this:

You better lose yourself in the music, the moment
You own it, you better never let it go
You only get one shot, do not miss your chance to blow
This opportunity comes once in a lifetime yo

According to this, the song rambles a bit and has only one rhyme scheme, but it’s missing where Eminem is placing his emphasis.
My line break looks like this. Here you can see that there are multiple rhymes and that the foot structure is complex (I would say nested).

You better lose yourself in

the music,
the moment
You own it,

you better never let it go

You only get
one shot,
do not

miss your chance to blow
This opportunity comes | once in a lifetime yo

It’s got a rhyme scheme – so even by traditional standards, it fits the definition of poetry.
The interesting thing about this question is that it is about ground zero of how linguists versus most other people view language. Rau’s two main objections to rap are 1) it’s not good standard English and 2) it’s not pretty. I don’t think they’re valid objections.
For instance Rau complains

First off, “lil” isn’t a word; it’s a lazy man’s version of “little” and the only time “lil” is acceptable is when we’re talking about Lil’ Abner. Secondly, “drop it like its hot” should be “drop it as if it were hot” and what does “it” refer to anyway? And why do I have to drop it? Why can’t I place it down gently?
And why does Fifty Cent refer to himself as “fitty” as if there weren’t another “f’” in his name? Since when did f and t switch places And why is the th sound now an f? Take P. Diddy’s lyric from his cover of Sting’s “Every Breath You Take.’ [Rau’s punctuation] He talks all about the “strenf” he needs. What is strenf and where can I get some?

My response – it’s not written in standard English, but some form of African American Vernacular English and the phrase “as if it were” is just not commonly used in that form. Nor is there a “th” – they’ve all become “f” as in “strengf”. Deal with it.
Or in more academic terms – you cannot rule out that a form is NOT poetry just because it’s in a non-standard language. If you did that you’d be ruling out bluegrass music lyrics which many people do feel is lyrical art (although that may also be a recent shift in attitudes). However most people associate art with prestige language so Rau is not unusual here.
Rau’s other complaint seems to be the lack of prettiness in rap.

When we get to poetry, I amaze them with Marlowe: “‘Come live with me and be my love’ . . . isn’t that pretty? See how the shepherd is wooing her?” They don’t seem impressed.

My objection here is that this criticism is based on a subjective judgment. Rau is expecting a certain romanticism found in older styles of English poetry, but is extremely rare in rap.
However, not all poetry around the world is “pretty”. Epic poems like The Iliadand the Táin (Ireland) could be damn blood thirsty at times, especially when heads were being chopped off or bodies dragged around the city. In fact many older European cultures separated war poetry (bloody) from other types of nature or romantic poetry (pretty). So again, I can’t use “prettiness” as a criterion for poetry.
That means I’m struck with “uses some sort of metrical system” (rhythm, rhyme or alliteration) as my definition of poetry.
Most rap seems to have a staccato rhythm, so it’s not surprising to me that romance is not a major theme (although L.L. Cool J. is an interesting counter-example). Rap seems to be the bloodthirsty and lusty form of the genre. And believe me, if you think rap is lewd…you haven’t seen Middle Welsh “erotic” poetry. The imagery there is about as subtle as the ads asking if I want to impress my girlfriend in bed more.
BTW – It’s not that the culture is lacking romance. Many R&B songs are very romantic and even “poetic”.
P.S. I normally like the Irascible Professor blog, so I was very disappointed in this column…but not enough to stop reading it.