Shelden seemed to want rebounds and went after them with some nice aggressiveness, so he's got the mentality of a rebounder.
Physically, the big asset that he has (and Leon) that BBD doesn't is that he has longer arms. He can reach for boards and loose balls, essentially covering more space than he takes up. BBD seems to take up more space than he can cover.
He looked pretty good on rotations, too, flashing, showing, etc.; his mobility and reach help there, too.
He looks pretty good. BBD is probably better, but there is no doubt in my mind that Shelden belongs in the NBA, and that he can definitely play-role player, of course.