And to say that "rappers don't sing" or that "rap music lacks instrumentation" is a poor argument, and here's why...
The former? Of course rappers don't sing; if they did, they would be singers, now wouldn't they? And believe me, with the exception of K-os and Lauryn hill, you don't wanna hear these guys sing...
Also, by that logic, genres of music with no lyrics whatsoever - Electronica and even Orchestrated - automatically suck because, well, there are no lyrics to sing.
The latter? That statement shows how the vast majority of you all don't realize just how versatile the rap melody can be. A song by The Game features a piano that only plays 2 notes. Simple, but effective. There's an old Wu-Tang song which uses violins for it's melody, and they go ballistic towards the end of the song (when the solos kick up). An old Ludacris song needs little more than a set of timpanis (sp?) in order to be faithfully recreated.
Subject matter is another big deal, as there are or can be songs chock full of references and metaphors (some better than others, of course). In fact, rap music probably has more references to video games than any other genre of vocalized music (nerdcore rap notwithstanding). Generally speaking, references can range from astrophysics, to contemporary art, to Aqua Teen Hunger Force, to Tekken 3. Listen around, you might be surprised.
It's past my bed time, so I'm out this piece. Oh, and merry dammit Christmas.