When I think of realism I think of the physics model in the game. the environment has to look real to the period its suposed to be in. The environment should set the correct mood of the game, the sound and lighting should also.
I like the way Unreal tournament looks compared to the cartoony way quake 3 looks if that helps at all.
Are 2d games pretty much dead these days, everything is going 3d and I dont see the point of it in alot of games (RTS type anyway)