Aside from all the other themes mentioned about rap, here is why I don't like it. The music, just from a musical standpoint, is sub-standard at best compared to its predecessors. Forget the racial component for just a moment, look at the structure of much of rap itself.
The vocals are often weak or nonexistent, and what about backup singers?
The instruments are often weak or nonexistent, strings, brass, and guitars have been replaced by a synthesizer.
Call me an old man, but when I listen to music produced by black culture then give me a band (an actual band) like Earth Wind and Fire any day of the week. I saw them last year and even in their 60s these guys still know how to put on a show. And what was in that show?
Actual instruments, including brass
A strong lead singer (Philip Bailey, you've still got it)
Backup singers
And of course some incredibly catchy beats and harmonies
When I compare this to thump-thump-thump, marginal or no singing ability, no backup singers, no instruments, then the choice to me is just plainly obvious. Rap to me is like fast food and Motown of old is like going to a nice restaurant. If you like fast food, all well and good, but please don't compare it substantively to what real "chefs" put into their music. Just like all food isn't created equally, all music isn't created equally either. Where I think folks get tripped up is that if you are white and cannot articulate exactly why rap music fails on so many levels musically it is entirely too easy to be labelled racist as a result. Because, naturally, there are some whites that don't like rap that are in fact racists, but that is just a subset of a greater whole that does not like rap music period.
The poetry in rap may be its best selling card, but come on folks, good lyrics are only one element of good music. Rap as a whole is just plain lazily slapped together compared to what came out before it.
I find the same phenomennon to be in true in modern rock as well, this dating back to the 1980s when the necessity of producing a cheesy video effectively put an end to the quality studio bands. When I saw Steely Dan in concert several years ago there were eleven musicians on stage, only two were singers. There were two different pianos, a three person brass section, etc. This focus on complex music seems to have largely evaporated.