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
The only thing I would disagree with is the "evil PCs" thing, but I have not once been in a game where there was one evil PC and it turned out even halfway decent for all involved; I banned evil PCs entirely for my home games for this reason. In the decade+ of DMing, it's never turned out well. >.<
That being said, I'm fine with an all-evil PC game, as long as they're not stupid evil psychopaths...which, unfortunately, is exactly what happens.
My own dealbreakers are a bit idiosyncratic, because in the last decade, I've only been a player for about 4 months. As a player, though, my requirements are:
1. No DMpcs. It's never turned out well. I also don't like it when DMs head-drag an absent PC; blame the last time I was a player.
2. The DM has to actually prepare his adventures and at least have a little bit of consistency in his world. This stems more from my last abortive time as a player; the DM refused to understand that everyone wanted to quit from day one because the DM could not understand that no one had a clue about his setting.
3. The DM's fun is more important than the Player's fun. It should be about equal, but it's important for DMs to know what kind of thing the players would enjoy, and to find the happy medium between them. This also stems from the last time I was a player; the DM had this story, and cared nothing at all for what we as players wanted to do for their characters or at all.
4. I'll echo the no-favoritism thing, as well.
5. Plot armor. I hate plot armor. Every PC should be able to die. No deus-ex-machina. If the players are clever, they should be able to save the PC that died via magic (usually).
6. If a DM has houserules, they should be laid out long before making characters. In addition, if a DM houserules something because it harms their plot or story but doesn't harm the player's fun, then that's bad, too.
As a DM, my dealbreakers for players:
1. When players don't bother to at least read the small things that I give them. Sure, I might hand out a decent amount of info before a game starts, but out of that info, I'll ask the Players to read maybe 15% of it when making their characters. There have been several instances where a player will complain about something that I haven't addressed, where I will then have to pull out the stuff I wrote for the campaign and point at it and go, "Your answer is here. It's been here for months." I got two games going; one, I never have this problem. The other, I can't get a couple players to read two pages, ffs. I hate that game, and I'm struggling to find a reason to keep running it.
2. No side crap. Don't be talking on your phone, on Imgur at the table, etc. Contribute. Have fun with the other players and go do stuff. Don't be a stick in the mud and don't wait for adventure to come to you; I prefer active players to passive players.
3. No loners or dickhead characters; the argument "It's what my character would do!" isn't an excuse because you made a character that was anti-social.
4. Make a character who is not a super-person. I
hate characters that are, "Oh, I have all these abilities and I don't want any flaws so I have to multiclass into everything!" It bothers the crap out of me; characters should be flawed, they should have hard times sometimes. There should also be times where you get to shine as characters, but the struggle should always be there.
EDIT: I forgot to mention, which should go up at #1 on both lists: ARRIVE ON TIME FOR CHRIST'S SAKE! God, I hate that. Be there at the time we start ready to play. We all have lives, and I am there constantly early just to make sure that everything is ready. You can do the same.
No, I'm not angry. Just disappointed in my last DM, who couldn't figure out that arriving 10-45 minutes late every session was not okay.
