OOC drama.
Lack of effort or chemistry from/with writing partners.
Army.
Traditionally, those are the big three for me. I've exactly zero tolerance for OOC drama, largely because I know how easy it is to get sucked in. I don't want to be that guy, so of I see it, I'm gone. If I start a thread and it doesn't feel like the other person is trying, or it just doesn't feel like we're clicking, posting becomes a lot more difficult. And, of course, when I do start something that's drama free and there's good chemistry, that's usually about the point I find out I'm going to be spending several weeks to a year someplace where posting is difficult. C'est la vie.
Here lately, I've been able to add depression and a few other mental health thingies to the list. Hard to write when you have no motivation for much of anything. Hard to write around others when you're reasonably certain real life experiences will bleed through and traumatize all involved. Or maybe that's the anxiety speaking. Eh, who knows?