Rap is one of the most absolutely diverse musical styles in the world today. Especially if your definition is wide enough to include, say, hip hop, dubstep, grime, ragga, and all the other awesomeness coming out of London slums.
Rap isn't consumed the same way as rock, in my opinion. It exists in a time and place, when the time and place are gone it's no longer applicable. It evolves faster than anything else.
That being said, I am far from a rap snob. I listened to Ne-yo all the way home today. Fuckin' shoot me.