Deal-breakers before the game starts:
System being used - is this a rule-set I'm willing to play? If it's 3e-4e-PF or a story-first system e.g. Burning Wheel I'm out
Realism or gamism - if fireballs aren't at least vaguely spherical instead of cubic, I'm out
People - do these seem like people I want to get to know, or get to know better - if no, I'm out
Deal-breakers as the game starts:
Are evil PCs banned? If yes, I'm out
Are PCs not of the player's own gender banned? If yes, I'm out (though rare, there's actually DMs who enforce this)
Is romance, attachment, jealousy, sex, etc. allowed between PCs? If no, I'm out (and there's DMs who enforce this too)
What's the advancement rate going to be like? If it's fast enough that I won't have time to learn one level's abilities before another lot get piled on, I'm out
Are the players allowed to quaff a few beer during the game? If no, what's the point - I'm out
Deal-breakers after the game starts:
Inconsistent DMing - a ruling made once is or should be a ruling made for all time
Favoritist DMing - is the DM blatantly favouring a player or character, or blatantly picking on a player or character? If yes, I'm out
Can the DM handle curveballs - if the players can't choose what to do next or the DM falls apart when the players do unexpected things and isn't willing to improve, I'm out
Player meta-gaming - if the other players insist on using player knowledge for things instead of character knowledge, there'll be a loud argument which if I lose, I'm out
Are the in-game threats real or fake - if for example the PCs turn out to be effectively immortal or plot-protected because the DM won't let them die, I won't be there long
Is the party run like a military unit where everyone has to follow orders and everything is planned down to the nth degree? If yes, I'll get bored quickly and then be out
Does the DM allow time for things to happen between encounters and between adventures? If no, see ya.
Lan-"this long list probably makes me seem somewhat fussier than I really am"-efan