Well, I haven't played the MP version of the full game yet, so I can't form any opinions about it.
But a mod with a class system would be nice. Kinda like DoD, where you pick a class and that class has a certain speed, a certain weapon, and certain abilities. As well as add a little realism to it. But I guess we'll see. Sometimes all it takes with this game is a server side mod like the realism mod to make it fun.
Probably one of the biggest let downs for me would be if there were no medics. Medics are so under used in games, and I think properly used they would add a lot to the game.