Kevan Barlow is the first I can think of where 90% of this board though the guy was going to be a monster, and only a few dissented. Boy was the mob mentality wrong on that one.
Marion Barber is a more recent one. He hasn't quite panned out to be the STUD everyone hyped, though he's been a bottom tier RB1 for FF purposes.
Tatum Bell was hugely hyped on this board, he was an absolute bust.
Koren Robinson was supposed to relegate DJax to an afterthought...
Cecil Collins, Ben Gay, Reshard Lee all had quite a following.
There have been a few cases where a minority hyped a player that the mob railed against... Boldin when he was a rookie, CJ3 last season, heck Edgerrin as a rookie and LT as a rookie - the majority here said they both would be busts - even FBG staffers, I think someone mentioned Turner - who I felt prior to his being drafted by SD had all the potential to be a stud RB.
There have been a few players, like Marshall, that maybe one or two posters like, and no one else knew about, that turned out very well. Brandon Jacobs falls into that group as well.
I've found, in most cases, when the mob hypes a player, they don't pan out as well as they are hyped.
The best thing to do with this board is use it for information, but not to let others make your decisions for you.