There is still good rap being made...it's just that unfortunately people would rather listen to T-Pain's computerized voice on the radio than someone like Nas or Lupe.

Though I do think it's funny that a lot of mainstream rap artists these days have #1 singles that go on for weeks (Flo Rida), yet their albums still struggle selling above 200,000k. Soulja Boy was surprisingly successful though. Never thought he would have another hit.
But since we're talking about mainstream pop too, I think Rihanna's music would be 100x better if they were sung by someone else (with the exception of Umbrella). Her voice can get hard on the ears when it comes to her ballads... *shudder*