I think the problem with those 'twitch deathmatch' games is that to most people, they're **** boring. There's only a very limited amount of things you can do, shoot people with a tiny variety of guns. And most of the time you're being repeatedly raped by a 'pro' player who's far better than you that isn't particularly fun. That's how I see those sorts of games turning out anyway, since I haven't actually played them. The game I enjoyed most was BF1942 (did't ever get BF2 because I couldn't run it when it came out and I never really did like modern shooters anyway). I enjoyed it because of the huge amount of options it gives you. Sure, you can play infantry for a while, but when you're sick of getting blown up by grenades you can hop in a tank and blast the little nade spamming **** into oblivion. Then when you're sick of being destroyed by planes, you can hop in one of them too and dogfight or blow up tanks. And when all that gets boring you can hop in a battleship and shell the **** out of an island.
I personally don't like level up systems or achievements. They are put in place to trick players into playing the game until they've unlocked everything, at which time they realise that the game doesn't really have any substance or replayability. I don't really like RPG elements in FPS games, but I do like a basic class system for a bit of variation. I haven't really enjoyed any of the newer FPS games, although BC2 will hold my interest for a while longer yet.