One that comes to mind that's partly matter of circumstance as much as actual expectation v bust: Lucca Staiger.
First, there was the whole sit-out year fiasco ("Will he or won't he be allowed to play?"). Had he arrived as expected, the hype-o-meter might have been more in balance.
Secondly, although touted as a key piece to the puzzle, I think by that point the program's framework/roster development already was shaky, and it was unrealistic to expect more than a 6th-man/sniper.
