Two things that bother me: ability to go through other players, and lack of friendly fire.
We'll start with the first one. Now, though this wouldn't be a particularly good argument, I'll say it anyway: I don't think I've played a first person shooter (or any shooter, for that matter) in which players can go through other ones. And frankly, it seems to work for them. Trying to melee a solid object is a lot less difficult than trying to hit one you can run through. Also, to me anyway, it makes the game look less polished. Being able to see inside of another player just doesn't look good. The other thing: granted, my programming knowledge is pretty limited at the moment, but I can't see this being to hard to do. Oh, and for the record, the enemies in campaign are solid.
Lack of friendly fire seems like a bad thing to complain about; apparently, it prevents people from being annoying and shooting their teammates for fun. However, I've found that not having friendly fire can make people kill their team mates as well; many times, I've had people walk in front of my rockets on purpose to kill me, and the same with grenades. Also, I've had people block my shots so that they can go in a steal kills. I know this sounds like whining, but I'm just trying to point out that not having friendly fire can be just as bad as having it.
Pretty much all my other complaints or suggestions have either been addressed by people on here, or by HVS, so that's all I have to say *cough* p_-_-_-_ s_-_-_-_-_ (I don't like the idea of being shot, so I won't say it explicitly).