Iverson was a big bust for two different teams, though it wasn't that surprising. He couldn't accept a supporting role (6th man or bench player), so his career is over.
Richard Jefferson is definitely a big bust - a rare miscalculation for the Spurs organization. He doesn't seem to fit with their team at all.
Al Jefferson has been kind of disappointing, taking a lot longer to come back from his injury than expected and also not helping his team win many games at all. I didn't expect the Timberwolves to actually be worse this year.
Rasheed Wallace definitely qualifies, although I think it's in part because people had higher expectations for him than they should have. With the benefit of hindsight, though, adding him to the team definitely wasn't as big a help as anybody thought it would be. It's pretty laughable now looking back to the beginning of the season when some people though Rasheed might contend for 6th man of the year.
I'm disappointed that Anthony Randolph hasn't really done much this season. More than a few people talked about him as a potential Most Improved Player candidate, yet he hasn't played more than 20 mpg and is currently out with an injury. A lot of that can be attributed to Don Nelson, who never plays young big men who can't shoot (he'd rather play Anthony Tolliver than Andris Biedrins), and insists on putting AR at center when he's really a pure PF. I was really hoping Randolph would get traded to a team that would actually utilize and develop his considerable talents. He's still super young so it's not too late by any means, though.
Michael Beasley hasn't made much a splash at all in the NBA so far, which is fairly surprising. He wasn't fantastic in his first year but he did show quite a bit of potential. I haven't watched the Heat much this year, but from what I've read and seen it doesn't appear that Beasley has shown much improvement in his game this year. That's got to be disheartening for Heat fans considering he was the #2 overall in '08.