They just lack a consistent defensive effort.
And when I say consistent, that means from the time the opposing team takes possession of the ball up until securing a defensive rebound. Second chance points murder them. Somebody other than Biedrins needs to BOX THE FUCK OUT because Biedrins is the one protecting the basket. You couple that with a lot of empty offensive possessions (turnovers ... Stack Jack usually), then you have a problem.
When they get after it defensively (use Celtics game as an example), they can play with anybody. When they don't (use Mavericks, Thunder games as examples), they couldn't beat UNC.
They need a Brandon Bass, or a Josh Smith type to start at the 4. They need a big body that can run as well as defend.