With the way the NBA is now, I don't necessarily consider draft busts a big disappointment, say a guy like Stromile Swift, a disappointment sure, but not a big disappointment in my book. Even though he was drafted #2, I don't think he had super high expectations by most people, nobody thought he was a sure thing, it was a big gamble, drafting him was like buying a lottery ticket, maybe this can be a million dollar pay off in 4-5 years, or maybe he'll be a bust, but that's the way the NBA draft is now, a lot more big gambles early in the draft. It wasn't always like this though, back in the '90's top 5 picks were supposed to be more of a sure thing. So for big disappointments (although I wouldn't consider any of these the biggest):
Joe Smith. #1 pick in '95, and pretty much the only award on his resume is All-Rookie team. Never an All-Star, never finished in the top 10 in any significant category. Never really established himself as an everyday NBA starter (started about 60% of his NBA games, but if you look at it season by season it's all over the place). Ended up being a solid NBA player over 15 years, but if that's all it says on my resume after being the #1 pick when 3 of the 4 guys drafted after you played the same position, and all 4 guys drafted after you made All-Star games/All-NBA teams I'm disappointed.
Billy Owens, #3 pick in '91, traded for an already 20ppg scorer in Mitch Richmond. Of the top 5 picks in '91, Owens is the only one to never make an All-Star game/All-NBA team. Never even whiffed the top 10 in any significant category. Again the only award on his resume is All-Rookie team. Had a solid NBA career over 10 years, but when you break up Run-TMC I expect big things. Big things which Billy Owens never came close to delivering.