There is a few big guys around the world that are tall and just lack the strength or toughness to compete in the NBA.
Luke Nevill is playing in the NBL and is 7'1" I believe, which is tall enough to occupy the middle, but just doesn't play aggressive enough to keep up.
Then you have the other end of the spectrum - DeJuan Blair - he has the strength and toughness, but is just a midget that plays Centre! lol