This is a rare occurance, but there was one player...one player who annoyed the living shit out of me. Playing on silver, and all he would say on the voice chat was 'aww yeah', 'you guys suck', and 'I rule, guys'. Not only was this shameless self-pandering and egotistical, but he was also the one I continually had to go out of my way to rez. Sure, he ended up in first place, but only because he was a vanguard that continually ran into huge groups and tried to take them all on, only to get downed by the last one or two of them, or get insta-killed by a banshee (seriously? to all vanguards/krogans, BANSHEES ARE NOT JUST ANOTHER ENEMY TO RUSH AT AND TRY TO MELEE TO DEATH...THEY WILL KILL YOU FIRST, YOU STUPID BASTARDS) and spend the next thirty seconds cursing out every other player in the game for failing to rez him, though by that point, I think we were all so annoyed with him that we were ignoring him. Even when I ended up essentially beating round 8 for the team because he was stupid enough to rush in mindlessly for the uptillionth time, he offered a begrudging 'well, you're not terrible', before starting to insult every other player in the game again.
Beyond that guy? Sure, there are things in the multiplayer that just piss me off. Adepts, Engineers running around with widows, for one. Honestly, the class is designed to USE THE DAMN POWERS. Don't throw that away for the opportunity for a heavy ass gun that does you absolutely no good at close range, without the ability to even get further away (like the tactical cloak with infiltrators? Yeah, it's not only for revving, guys, it's for getting the hell away from the big nasties that like coming up behind you.)
Krogans and Vanguards that rush banshees, brutes, or atlas mechs on silver and gold... Honestly, just because it worked on bronze does not mean you're going to be able to headbutt them into submission at this point in time. Most likely scenario, you're about to get a hand stuck through your chest and die. Am I going after you? Hell no. I'll stay back and work on them from a distance where they can't just kill me instantly. Honestly, you get more points if you don't get killed... It just works that way.
The one thing that really gets to me, though, no matter what class is doing it, are those players who think they're so accomplished and godly that they stay as far away from every other teammate as they can, because they want to get the highest score. If they get downed, there's no way that anyone else on the team is going to run through the two ravagers, two brutes, and a banshee to rez them because honestly, that's suicide in its own right. I've never seen a player good enough to survive on their own when they draw the pack towards them on gold. It just doesn't happen...maybe once in a long...long time.
And then there's the people who quit mid game. I know, from time to time, the system just disconnects you, but sometimes it's just blatantly obvious that they do it intentionally. Namely, when there's one survivor on the team, and they quit without even waiting to see the end. Not only does this rob you of the rewards if they do die, but 30-40% of the time, a single person can whittle down the remaining enemy one by one by moving constantly and not getting surrounded. That's worth it, in my book, and I've always been proud of the people able to do that and bring their team back at the end of the round, even perhaps eventually winning the entire match. Does that person feel badass? Hell yes. I know I do, the times I manage to do it. Seeing that 'Full Extraction' flash up on the screen when only a few rounds ago, you were desperately scrambling to survive is a very empowering feeling, especially on silver and gold, when two banshees and countless brutes come rushing towards the extraction point with your blood on their minds. Watching that counter tick few the last few second while spamming bullets and powers in a desperate attempt to make it is a heady feeling... It's one of the main reasons I enjoy the multiplayer so much.
EDIT - Oh, and human vanguards? Don't use nova in a group of three or more enemies... IT REMOVES YOUR SHIELDS, dammit... Not something you want when you just used your biotic charge to get into danger in the first place, and can't use it again for the next few seconds...