I play and DM roughly equally, so from both sides of the screen, starting with as a DM:
- players who care too much about their numbers - just roll the dice and let the results help define your character
- players who think everything is there to be killed or looted
- players who moan about someone else being weaker/stronger having better/worse items
- players who constantly interrupt - a bit of chatter is fine, but sometimes you need to be willing to keep mouth closed and listen to what someone else has to say.
(though I would not walk away from these type of players immediately, I would try to change their attitudes 1st)
And as a player - and yes I am old, grumpy and more than a little fussy:
- playing with precise measurements on a grid, this is a big no-no for me and I would not even start in any game with a GM who insisted on it. I don't mind tokens/minis being used to give a general feel of where everyone is, but that's as far as it goes.
- enforced, unrealistic balance. I've played a 1st level PC in a 5th level party and I don't have a problem with it, if I survive 4-5 sessions I will mostly have caught up anyway!
- point buy - just let me roll
- standard array - even worse than point buy
- a DM who changes core rules because he believes he knows better - you DON'T.
- other players who try to munchkin as much crap as possible - just play official races and classes, RAW, and be done with it.
- a DM who wants me to play an Elf
And other things which I dislike, but which by themselves would probably not drive me away:
- technology at the table - pencil, paper, eraser, dice - you don't need any more
- Paladins - just because
- everyone else picks Elves - sorry, that makes you all powergamers.