Rail road plot...first I understand you made the dungeon, and we will explore it, but if you have you 12th level bad guys picked out at level1...no thanks
At the risk of going off topic, having enough of a story outline planned out to have villains ready 12 levels ahead is
not automatically railroading. There's
plenty of room between "complete sandbox" and "linear railroad" for a good, plot-intensive campaign that still allows substantial player choice.
Back to topic...
There are certain games I'm not interested in playing, but if a group of good friends really wanted to, I might at least give them a shot.
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.
I won't play with people smoking inside. I used to, and I can't tolerate it anymore.
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.
I won't play with people whose only goal in playing is power-trip fantasies, or venting socially repugnant fantasies. If you want to run an evil campaign as a change of pace, that's fine. If you want to do it so you can linger over your description of rape, I'm gone.
There are probably others, but those are the ones that come to mind at the moment.