Erden's rebounding and defensive shortcomings go hand in hand. Late rotations, switches on pick & rolls, etc also leave him out of position for rebounds. Many notes above about strength, and other issues are also valid, but I think that with playing time he'll get it.
He brings good foot work in low post offense, good IQ, and willingness to work hard and play through his injuries. His shot blocking, which hasn't been mentioned much above, is exceptional as it is approaching 2 pg in the last five games when he's had consistent playing time. I have been pleasantly impressed with "MR IRRELEVANT" who I thought would be working for the other Ainge.