No evil PCs. I don't want to pretend to enjoy gaming with someone when they go on about how they enjoy <insert evil act here>. In addition, in my experience evil characters end up stabbing other PCs in the back at some point.
Consistent non-joke world. I'm OK with humor, but when you have a town name BobTown where everybody is named Bob just because you thought you were hilarious, I'm going to start looking for a new game.
Nonsensical house rules. It's OK if paladins and gnomes are banned in your world, just give me some justification.
No rolling for ability scores or HP.
No grim-dark/depressing campaigns. I play to have fun with friends, blow off steam and enjoy myself. Conflict and hard times are fine and even expected, but if the entire campaign is one depressing event after another and a futile fight against the overwhelming darkness we cannot hope to defeat? No thanks.
Inconsistent rulings/favoritism. The DM needs to make rulings all the time. But if they decide that their girlfriend can hide behind a blade of grass because her PC is skinny for a half orc and my halfling gets spotted half a mile away by a blind guard because I got "only" a 20 on my stealth check we're not going to be gaming together long.
Incredibly dumb house rules. I had a DM that had a 6 sided dice, with each side having a body part. Head, torso, leg and so on. When the enemies crit, he would roll the dice to see which body part got lopped off. Arm? You get your arm chopped off. Leg? No leg, you have to crawl. Head? You're dead. Torso? You're dead, stabbed through the heart. At 150 HP when the kobold stabbed you for what should have been 6 points of damage? Too bad, still dead.