I dislike most rap since it promotes senseless violence, sexism, and the use of material goods or "bling" as a status symbol. These people claim to represent the impoverished ghettoes yet live it up in their mansions with their millions of dollars (which could easily be used to improve the conditions of those impoverished ghettoes, by the way).

Also, in general, rap songs don't have much of a real constructive message to express to its listeners. A lot of it is simply praise of intoxication, prostitution and material wealth. When they do try to address a real issue that has some significance, for instance the impoverishment of the ghettoes they claim to represent, they will only try to portray how bad things are while completely ignoring potential solutions people could take to fix the problem. Even if they had something thoughtful and insightful to say, the lyrics are spit out so fast that the listener has almost no time to reflect on it.

I have found a few exceptions to these general trends, but they tend to be bands which mix rap with some other music form, like Rage Against the Machine (rap/metal) and the Kottonmouth Kings (rap/punk).