At this year's GDC former Game Director, Jay Wilson, admitted that Diablo III's Auction Houses "really hurt the game."
Apparently a few assumptions were made about how gamers would use the Auction Houses (both the real money, and in-game currency versions), with Blizzard expecting only a small amount of gamers to take advantage of the marketplace. In fact, nearly all players used it at some point, with almost half of them using regularly.
According to Wilson (via Joystiq) this made money the primary motivator in the game, more even than taking down Diablo.
"I think we would turn it off if we could," Wilson said during his GDC talk, before admitting it wasn't that simple, with Blizzard unable to accurately gauge the way players feel about it. He did, however, admit that the team were working on a solution, although he didn't provide any details.