I've never understood campers. It's just like... come on dude, you've paid £40 for a game and youre just going to spend your time sitting there doing nothing? On Battlefield (the only MP shooter I actually play), I'm always moving, even when playing Recon (I only play engineer and recon, which is really similar to ME3MP where I only really play Infiltrator and Engineer). I'll take a shot, and move on from there. No point in hanging around. Means there's less chance an enemy will find me. My favourite past time, though, is killing campers by getting in a helicopter, spotting one, jumping out and parachuting, landing behind them and knife them. Because I want them to look into my soldier's eyes and realise that camping doesn't work.
Yep agreed, although I guess some people just want the easy kills without the risk and don't really care to improve. I've mostly played Battlefield myself but I roll all the classes though depending on the situation. My favorite is the hyper aggressive recon because you can shine in both short as well as long range with the bolt action/pistol combo and with some c4 and spawn beacons, you are a real asset to the team.
But I change it up depending on the map and gameplay, assault class with an AEK or Ace rifle for infantry maps where I can also revive teammates as well as have a generous supply of grenades. Or a support class machine gunner with C4 and ammo boxes if no one else is running that. I go for engineer and recon on bigger more open maps since both of them can take out vehicles and engage in longer ranges.
Also I use the vehicles like helis and tanks whenever I can get my hands on them. You can really annihilate campers with vehicles especially the little bird helicopter and what's more embarrassing than running them over with a tank.