[Morgeruat]

Not sure if his has been covered yet, but as a DM you have to know when to back off of the rules to tell a good story sort of tying into the "3 stages of GMing",

first the noob where you just let anything go because you're only interested in the fun, second the rulehound because eventually you get tired of #1 and want to bring the game back under your control and not under the control of now godlike PC's instead you overcorrect and rely too strictly on the rules to make the game, and 3rd the mix, you rely on the rules to establish a common ground, but they can be ignored or enforced as necessary to tell a story that's fun for all.

As for things that annoy me, disruptive players who are too busy doodling, looking at stuff online, chatting, etc to play the game. Players who just don't pay attention and miss key plot points, or players who do downright stupid things (bashing down a wall with a magical sword and ending up breaking both), and players who take the game too seriously, characters die, saving throws fail, monsters get lucky, terminal velocity lives up to it's name, this stuff happens, grab the dice, grab some paper, and roll up another one.