I don't get that he's bad defensively. I guess I haven't zeroed in on it, but he can't be that bad.
Well you are correct. He does a lot of things well. The other poster was raising the point that he plays back on his man, particularly out front and he does not move laterally quickly enough to block penetration into the paint. I've seen this myself and frankly it doesn't happen as frequently so I think its a reaction time issue as opposed to lack of quickness. He doesn't want to fall for the fake. His interior defense has improved and his weak side help (rotation or whatever you like to call it) is better. Like most shooters, he seems to be able to anticipate the ball for rebounding.
He's much better. He does give up that lane too easily and too often, however. I'll also add that this has led to fouls as folks try to help out and end up reaching and swiping at shots in an attempt to block them. That's on the helper though. Must defend straight up at the goal.