You know what the biggest annoyance is to me?
Backseat driving.
In particular, back seat gaming. you know what? you think you can do it better, go do it yourself, but on your own frakkin' time, not mine. This is the main reason i avoid doing single player between rounds at LAN parties, because there are always a dozen ready people saying,"no get the sniper" and "Quick, stick a grenade to him." it realllllly get on your nerves after 3 minutes. You Know, there's always the pleasure i look for in games of figuring it out for myself, not having some jerk thoughtlessly blurt it out.
The one place that should avoid backseat driving at all costs is puzzles. yet oddly enough, this is where the most backseat driving seems to happen. for example, in LOZWW there are so many great puzzles that are so easily ruined by backseat gaming. If you ask someone for help, that's one thing, but some one just walking into the room and saying, "oh, that goes there" is another thing. seriously, just do it later. if you enjoy solving puzzles, the other person might enjoy them too, so don't spoil the fun for them! and in a game like FFXI, help from a linkshell is fine. a friend in real life saying which job you ought to level now might be different from what mood your in.
So, to all you back seat drivers out there, kindly stop, and if can't do that, kindly go crawl in a sound proof windoless room, where you can backseat drive all you like.