One of the things that sets this game apart from other Superhero MMO's is its open end-ness (if that's even a word) your ability to customize your hero anyway you want, and not be pigeon held into a role.  Cryptic was considering introducing a "class" system into the game, mostly for resolving balancing issues .  It looks like the fans managed to get their argument though to the designers.

Its always nice to see when the fans and forums contribute to a game design.  The designers at Cryptic where rolling around the idea of creating "classes" of heroes for Champions Online, which would be a lot like the "archetypes" in City of Heroes.  I am glad the Champions online community talked them out of it.  One of the key features of this game is the level of customization you have over your hero, powers, costumes, arch-villain, well everything.  I think they would have made a serious mistake by introducing a class based system into the mix.  While I can see how a class system would make balancing the game much, Much, easier.  it would take away from this level of customization that the fans are expecting. 

 "Despite all of the complications and dependencies, at the end of the day it's the user experience that matters. A gamer typically doesn't really care how or why a game gets created: they care about if a game is fun. "Fun" is a highly subjective term, but most game designers get into the industry because they like games and want to create a fun game experience for other gamers. In short, a game designer wants to provide the most fun game experience for the most players."

 "In short: thank you for all your feedback to our game design articles. This is ultimately your game and we want to make sure we meet any reasonable expectations you might have. While it's impossible to please everyone, we'll do our best to review all your ideas and evolve the game design where we can."

http://www.champions-online.com/dev_blog/evolution_of_game_design_rethinking_archetypes