I have to say the most disappointing players to me over the past few years have been T-Mac, Steve Francis, Eddy Curry, Jermaine O'Neal, Yao Ming, T.J Ford, Elton Brand, Baron Davis, Allen Iverson and Gilbert Arenas.
Most of you will probably disagree with me on a few of these:
Many of these players' problems can be directly attributed to injuries of which many of them didn't have any control. In the cases of T-Mac, Francis, O'Neal and Brand this is certainly true. However, I think with Curry, Ford and Davis they are simply lazy and just didn't live up to their potential.
Iverson and Arenas are different because their attitudes and behavior has led them to disappointing careers. I can't really complain about the numbers they've put up but their actions (ie. refusing to accept a bench role, complaining, chemistry issues with teammates, bringing guns in to the locker room, etc) are too much for me to ignore. These guys could be great professionals but now neither is contributing to a NBA team.
Ming on the other hand has had great success though he disappoints me because of his lack of dominance. He's had a pretty good career but has never performed the way everyone knows he's capable of. Now his injury may end his career. I don't know what it is about him but he's never been what I thought he could be.