A lot of the things that people have mentioned would annoy me. There are lots of ones that others have mentioned .
From Terwox:
# DM's who play favorites, give certain PCs tons of special powers, while wantonly killing everyone else (man, I despise that.)
# People who think D&D sucks and that they can write a better system. This is pretty common among people I know... and they're all wrong. (see this a lot of thison these and other d20 boards)
From Zappo
Players who, when faced with something strange but not terribly important, assume that the DM either made a mistake or is deliberately fudging, instead of going out and investigate.
From Chupacabre
2) Players who don't know basic info about their feats, spells, or other abilities and have to look it up over and over.
From Force User
~I've griped at players because they were late to the session, or showed up without dice, or pencils or a character sheet. (I h8 it when players do this)
From William Ronald
* DMs who have no sense of campaign balance, and will have characters of greatly varying power in the same party -- and power being the only thing that truly matters in his game. (Santa Claus DMs)
* DMs who seem to center everything in the campaign on their pet NPCs.
* DMs who railroad characters. (story-line Nazis)
From Herpes Cineplex
* When the GM just sits there and says "So what do you do?", as though the entire game world has absolutely nothing interesting happening in it that should be called to our attention. Encouraging PCs to pursue their own goals and be proactive is fine, I'm just irritated when a GM doesn't bother to prepare anything or come up with any non-PC-directed plot, preferring to give the players the "Fine, your characters can just sit there until they decide what to do" routine instead. For god's sake, if you're going to run a game, run a game. (I had a DM like this, this was soooo irritating, what’s more he thought he was God’s gift to DMing)
* I'm overly "helpful" with other players, piping up with suggestions without being asked to. It's particularly bad when someone is playing a social character but seems to be fumbling an in-character conversation, and I'm trying really hard to stop doing it.
From Panthanas
I'm surprised no one mentioned meta-gaming. It annoys me to no end when players use knowledge their characters wouldn't have a clue about.
And now from me
*Cheating…whether it is with initiative or stats (I rolled those, honest!), I can’t stand dishonesty.
* Counting….players are like we’ve done about 75 points of damage. This guy should be dropping soon.
* I’ve got a player that her dice always falls of the table about 80% of the time. She insists of rolling right near the table edge (cause anywhere else is bad luck). It is sooooo irritating.
* DM competition, one DM we had was super competitive and would put down the other behind his back and try to turn the group against the other DM. This caused a split in our group and some friendships lost.
* Whining….period….h8 it
And stuff that I do that’s annoying
• I DM a lot so I have to be careful when I play that I don’t take over and try to be the Player/ DM (Oh that’s not right, the spot DC should be….)