Defense in 2K is stupidly hard in general. I'll force an offense into a tough shot, and then suddenly David Lee will turn around and hit a 20-foot fadeaway jumper with a hand in his face.
As far as the block animations go, I think they lead to too many fouls, as instead of trying to block the dunk, the player will swing his hand wildly and end up slapping the other player, leading to one and-one opportunity after another.