Personally I think that some of the suggestions people have been making *could* be added into the game without making it too off balanced. For example the way in which Unreal Tournament (and several recent Unreal Engine games with multiplayer) made use of "mutators" that were slight variations in the gameplay that could be switched on/off and combined to throw in some variety to online games.
Stuff like "realistic damage" or "limited carrying capacity" might be easier done that way. So that people who didn't like them could just skip using them, but those who did, could. Rather than making single player and multiplayer carbon copies of each other, I think Raven/LEC realizes that there is a distinct difference in the two gameplay modes. Now perhaps continuity is a big concern in many people's minds. Ideally then, Multiplayer would be balanced and single player would have many of the same conventions carried over.
But then, I admit, that after playing the single player campaign of any game, then switching to multiplayer, I basically have to learn it all over again anyway, since playing against a few AI and going through scripted objectives is different than playing against live opponents, with different objectives.
So to sum up, I'd like to see some of the better ideas used as "mutators" (options that can be toggled in the menus by the server admins) if possible.