To be able to rap, you've just got to be able to talk like a black guy. That in itself doesn't make rap suck, as roaring like a cookie monster isn't particularly difficult to pull off either. My problem is that the focus on the lyrics/flow w/e mean the tracks are usually repetitive and undeveloped, not to mention derivatively reliant on samples. I'm sure that doesn't hold true in every case, but rap seems to have an unusually high shit to quality ratio.
The other thing I dislike in rap is that rap coolness = one's ability to brag about how cool one is, which is pretty much the opposite of actually being cool. That's probably more true of the mainstream, but nevertheless it's so pervasive that it puts me off exploring further.