I write the following assuming I'm looking at joining a campaign intended to be (or that already is) long-running. Most of the below does not apply to one-offs, con. games and tournaments, playtests, etc.
Dealbreakers that would send me packing on reading the game intro:
- a system other than D+D or a very close variant. See ya.
- may only ever play lawful and-or good characters. See ya.
- must always optimize character to the fullest extent. See ya.
- no character death and-or no chance thereof. See ya.
- no in-party conflict allowed. See ya.
- no in-party romance allowed. See ya.
- no Humans, Elves or Dwarves e.g. all characters must be Tieflings. See ya.
- starting higher than 1st-level unless I'm entering an ongoing campaign where the PCs are already established at higher levels. See ya.
Things that would almost certainly send me packing after a greater or lesser amount of time once the puck was dropped:
- must always be part of or follow the party plan, no deviations. See ya*.
- the story is on a railroad with no forks or deviations allowed. See ya.
- limited or no turnover in players and-or characters in the long run. See ya.
- gaining a level more often than once every few months. See ya.
- playing less than twice a month on an ongoing basis. See ya, if there's another game going that'll have me.
- no depth to the game world, story rushes from combat to combat and adventure to adventure with no detail given in between and limited or no chance to interact with the greater game world. See ya.
- none of either the DM or the other players entertain me. See ya.
- characters who do nothing get the same ExP for something as characters who took the risk. See ya*.
- unfair treasury division by party. See ya*.
- too much thinking (planning, puzzle-solving, divinations), not enough action (exploration, battle, tension). See ya, after attempts to get things moving.
- DM unwilling to or incapable of dealing with a split party. See ya.
- players incapable of divorcing metagame knowledge from character knowledge e.g. the player knows we're fighting a skeleton but the character does not thus the character wouldn't automatically pull out a bludgeoning weapon. See ya*.
- no sense of humour, everyone takes it too seriously. See ya.
- DM rulings that are inconsistent within the game and-or game world or that contradict precedent set by an earlier ruling. See ya*.
* - probably after one or more long loud arguments either in or out of character.
This probably makes it sound like I'm a far pickier player than I really am. But I am chaotic as a player and sometimes evil as well, and this comes through in my characters more often than not. I like to think of myself as an entertaining sod to play with, and I somewhat demand entertainment in return: it's only fair.

And I expect to be able to laugh at the game, and at myself; I expect characters not only to be allowed to do stupid or gonzo things but to then go out and do 'em. And a sense of whimsy from the DM now and then never hurts either.
Lan-"whaddya mean I'm not allowed to attack the gazebo"-efan