The real problem I have with rap is not the sound, but rather the lyrics and overall message. I'm not a prude, religious, or "save the kids" nut, but I'd say 98% of the shit that is produced these days is awful to listen to if you use your brain.
It's often hateful, racist, misogynist, elevates "thug life" and crime, the list goes on. The days of classic groups like Run-DMC and the Beastie Boys have passed, and in it's stead we have Kanye and Dre who are proclaimed "artists" because they use the medium for so called "social commentary" wrapped up in abhorrent lyrics (that nearly always fall into the aforementioned tropes).
[I should make a caveat for a guy like Macklemore who actually is trying to preach something worthwhile.]
/end rant