Tyrus Thomas and Ron Mercer are two that come to mind as players i thought would be great but ended up doing about nothing.
Yes, Tyrus Thomas for me. I must have written dozens of posts saying he was going to be the next great thing, based on how he dominated defensively at LSU.
I'm sure there were some others, although can't recall at the moment.
Ah, Dante Exum. I thought he was going to be really good as well. Not sure how much injuries derailed his game, but he has had a disappointing career thus far.