Most of the frustration about MvC3's roster was based on the idea that the game's release was supposed to be this big event - 10 More Years, and all that - and yet Capcom and Marvel alike were "wasting slots" on irrelevant characters.
And yes, there was a little backlash against the competitive MvC2 players - characters like Tron (heh), Storm or Akuma caught some flack because some fans were convinced they were picked over possible new characters due to how heavily they were used in tournaments. Of course, some people were also upset that characters like Psylocke, Spiral and Captain Commando weren't making a comeback despite the fact they were tourney staples. And both of those reactions are only a small part of the overall dissatisfaction fans had towards the roster before the game was released.
At least, until Sentinel was announced. 10 More Years went from a battle cry to a death knell overnight. And he was one of the last few characters Capcom teased, too, so that pissed all over the event hype that people were enjoying.