For me it's the lag, has been since day one, I honestly couldn't tell you how many times I've killed someone only to have them kill me somehow then drop dead 1-2 seconds later with NO other people around. Another thing would be the people that only use one weapon the entire time, I've seen so many people that just run around using only the shotgun and nothing else the entire match, it's not that it gets me mad, because I can kill them before I'm in range, I guess it just seems lazy to not switch to a primary with some actual range depending on the situation. i don't care for the crucible really, but if I need extra xp from bounties i may venture in there to do a few quick bounties.