Inactivity, lack of interest/participation from players, people over-committing themselves when they have busy school/work/etc schedules - this one killed off all of my roleplays for months on end, and really pissed me off. Surely you must have some warning or idea of how busy you're going to be before you commit? 
Lack of GM interest as well - I've been guilty of this myself recently, but seen it happen to really good games from others too.