Players who constantly try to cast stasis on armored targets (it didn't work the first 5 times you tried it so chill)
Players who bring Armored Compartments V to gold games
People who mindlessly spawn nuke during the first few seconds of objectives (barring targets)
Cheap randoms who go equipmentless on gold
The entire Geth faction
Rio
Condor
Reegar Carbine
Biotic/Electric slash spammers
Seeker swarms
Platinum pugs (at least nowadays, months ago when I used to do it frequently I became acquainted with some very awesome people)
The fact that players are visible to enemies while loading into games in progress. Such bullshit.
Cryo explosion sound
Dagger ammo boxes
Host migration
Husk hugs
Spectator mode
"Origin is not running. Mass Effect 3 will now close"
Surprise turrets
Wave 3 targets
Wave 10 devices
The fact that being off-host severely lessens the effectiveness of many things (siege pulse, most vanguards)
I've never cared at all what kits/builds people use, no matter the cheese. That doesn't annoy me; it's peoples' nonsensical habits that annoy me.