Vanishing and inconsistently posting GMs. That's one of the main reasons I don't endeavour to join other RPs and only work on GMing my own, because I'm confident in myself being committed to see the rp through and at the posting rate specified. I'm sick and tired of the number of rps I've participated in that have died or failed and not a single one of them ever reaching completion. 

Players who join in the midst of an ongoing rp, refuse to take any constructive criticism when they're doing something wrong/inconsistent with the rp, being asked to fix it/clarify other points and then them bursting out in a tirade of anger, snottiness and ego, presuming to know how the rp should be run better then the actual GM. Naturally, those players don't last long. 

Players who bitch at me about other players within an rp in pm over absolutely nothing is always terrific fun too.