Veron was a very, very good player in his day, he just didn't fit in for a strange reason. Berbatov was a very skilled player who didn't have the right mentality or work rate, or physical attributes to succeed at United (it was obvious before we signed him tbh).
Fellaini is just bad though. He's not talented, he's slow, he's not very good defensively, he's not a good tackler, he's not a great passer and he's not a hard worker. Pretty much the only thing he's actually good at is heading the ball, bringing down long balls and fouling players (which isn't a good quality to have). He has a decent first touch too. It was painfully obvious to me and a few others he'd be shit here, because he just isn't good. It isn't a case of flopping, or a player not having the right mentality. He's just not very good. (Saying that, he was okay for Fellaini standards today, I don't expect him to be capable of much more then that though)
Our best midfield is probably Carrick and Jones. That's a little depressing when you think about it.