He just measured in at a **** hair under 7' barefoot, over 7'1 in shoes, 250 lbs. He's 20 years old and a hell of a physical specimen. He produced at least as well as all of those guys even though they're a few years older. Leonard barely played his freshman season but made huge strides his sophomore year. I think from a physical stand point you can't deny that he has more potential than those other guys, but looking at his age, his production, and his improvement why wouldn't you think he's going to keep getting better?
None of those guys are going to get drafted on the basis of being a guy you dump the ball into on the block and expect them to score. These days NBA centers are largely the anchors of a defense. If you're talking about guys you project as that type of impact player on defense, the only other guy I would think has a chance of being that kind of player is Melo. But Leonard's chances of becoming that kind of a C are much better IMO. He could be a Tyson Chandler kind of player on defense.
Of course, any team who drafts him would be wise to protect their investment by making sure he gets a lot of coaching. Hes a long ways off but if a team likes his work ethic, believes they have a coaching staff who can bring him along, and is patient with him they could be getting a really high level C.