To many people, "fair" occupies the same mental space as Santa Claus and the Fallout MMO -- it's nice to believe in, but it doesn't really exist. Balance in an MMO is hard enough without adding in standards of fair play, and in any environment more complex than Team Fortress 2, there will be some things youu consider blatantly unbalanced. (Even in that environment, actually.)
But there are always some things that strike you as just being inherently not fair -- things that on a conceptual level make the game less fun, especially in PvP. Stuff where you find yourself blaming the mechanic even when something is your own fault. Stealth is a classic one, as many players find the idea that you could be killed by a heretofore unseen presence not remotely fair. Another classic is any form of crowd control -- few people are pleased at PvP matches consisting of their character being unable to act while other players unload on them. What about you? What mechanics do you consider inherently unfair to play against?