I won't play in a game where role-playing (things like "speaking in character") is forbidden. I'm not interested in a minis tactical game masquerading as an RPG.
There are games like that? Wow, never heard of banning roleplaying at a roleplaying game. Yeah, that would deal be a deal breaker for me.
I won't play with people smoking inside. I used to, and I can't tolerate it anymore.
Yeah, I've got a few gaming friends that smoke, but they know to step out for smoke breaks.
I won't play with people who are drunk or stoned. A beer or two is one thing, but if it starts impeding your ability to take the game seriously--or, alternatively, causes you to take it too seriously--then it's impeding my ability to enjoy the game.
Yeah, I've gamed with alcohol, and I had a GM once that was a far better bartender than a GM (her games were mediocre at best, the drinks she served were incredible), but we weren't getting drunk, but I personally don't want to be where illegal drugs are in use (I don't want any trouble).
For me, the real deal breakers:
1. GM's who don't know what they are doing, and aren't trying to learn. We were all novices once, but I've played under a few GM's that didn't know the rules, just made up new rules to replace them as they went, then got angry with you when you didn't know their rules they just made up or that you tried to educate them on what the rules were. This also goes for GM's that are trying to make the game into a novel (run away!), let their SO play some uber-PC clearly superior to the rest of the party (or have a pet DMPC played by their SO that runs right into "Mary Sue" territory), or is running on such a tight railroad that there really is no way off and you might as well be handed a script to read from.
2. Games that go way, way too late or start way, way too early. I had a GM once that ran games that were supposed to start at 2 PM and go until 10 PM, a decently long 8-hour session. The problem is, she started as soon as most of the PC's were there, or she felt like starting (which typically meant she'd start the game at noon or 1 PM), and she wouldn't stop until she felt she was done, which meant the games often dragged on until after midnight.
3. Nothing happening. I've played in games where the GM spent more time focusing on petty inane parts of life than adventuring. I'm not there to spend an hour roleplaying talking with a baker about the weather while waiting for the mornings bread to finish baking, consoling a random scullery maid whose daughter died of plague, or being chatty with the NPC cab driver on a long drive (yes, I've had all of those happen!). If there are sessions where I spend hours playing and not a single thing happens with regards to plot, something is critically wrong.
3. I'm no "canon lawyer", but if a GM is going to deviate seriously from the established setting if it's one I am very familiar with (Forgotten Realms, Planescape, Star Wars, Star Trek. . .) I'd appreciate plenty of advance warning, or if the GM doesn't know the setting well and is changing things because they are sticking with what they know and I know the canon better, I can deal with it, but I'd at least like to know what to expect so I don't go in talking about Spaarti Cylinders with a Star Wars GM that never read the Thrawn trilogy, or talking about Karsus's Folly with a Realms DM that doesn't know anything about the Fall of Netheril.
4. I expect the GM to know that gaming is a hobby, not my life. If I have a family emergency or something work related come up, I may have to bow out for the night, and don't take it personally. I enjoy gaming, I like gaming and talking about gaming, but I won't sacrifice family or career for it.