I have a few that are almost equally as horrible, but I suppose my biggest is communication (or lack thereof). Whether life gets busy or not, I feel like if someone doesn't communicate with you one way or the other, there's bound to be problems. And no, I'm not bitching about one specific person. Over my 10+ years of roleplaying, I've noticed this get on a lot of people's nerves. And equally so, I've seen it kill many roleplays. some of them I was in and some were roleplays that shouldn't have died so quickly.

I just think it doesn't take much to write an "I can't post right now" message to whoever is the GM of whatever roleplay in question.