Fran Vazquez ought to be on this list at the very top. Everyone had him pegged as 'the next great European big' but he didn't even make it to this country.
In all seriousness though, it might be a bit of a gamble to say it but I think it's a fair call to put Jermaine O'Neal on this list. Some guys I'd also put in for consideration...
DeMarr Johnson/DaJuan Wagner -- These guys were hyped HUGE in high school and even did fairly well enough in college to get drafted pretty high. Then they just fizzled out. Johnson through a series of moronic accidents, Wagner through some issues involving a defective colon?
Channing Frye -- Remember when this kid started out great, and really made a good early shot to challenge for ROY consideration?
And probably the best of all, right up there with Kwame Brown and the likes, not for the poor quality of what he actually achieved but because it was so much less than what was expected of him -- Christian Laettner.