1. Glitching- People playing Gears of War online love to exploit the game's issues by crab walking and using a shield along with a shotgun. Makes me very angry while playing.
2. Playing a mature game online as a little bratty kid- I can't stand people under the age of 12 who get on Xbox Live and think that they're cool for screaming and annoying the crap out of everyone.
3. Abandoning teammates- This is especially bad in games like Left 4 Dead where cooperation is really important. People will just run off and forget about everyone else in the game.
4. Kill stealing- This tactic will drive any gamer mad and it is especially common in Gears of War Execution which requires that you get close or use explosives to kill someone.
5. Quitting a game- This will ruin any team deathmatch because with an uneven amount of people on each team, it takes away the satisfaction of a victory or adds frustration.
6. Always rushing for the power weapon- Some matches feel like mini-races because instead of trying to kill each other, the winner is decided by which team has the rocket launcher and which team does not.
7. Taking a game too seriously- When you get too into a game which usually happens with a long amount of playtime or overconfidence, it not only makes you feel like crap but everyone else just finds you to be an annoying jerk.
These are 7 things that people do (and sometimes even me) that will make the game I am playing less fun online. Are there things not in the list that will anoy you that I forgot to mention?