Imagine you learned chess solely by playing against a chess computer. The computer plays quite well, but you learn by playing him often, and get bored of playing such a predictable opponent. So you decide to go to some place where other people play chess, so as to be able to play against more intelligent opponents and to have more fun.
Eager with your new plan, you turn up at the place so early, that there is no other human around. So to pass the time you start playing against your portable chess computer again. You're half through the match when the first other player arrives, wipes the computer's pieces of the board, and puts up 16 pieces of his own. Only you're down to 7 pieces already, and with 16 against 7 pieces your opponent wins easily.
You come back for a second game against the same opponent, this time 16 vs. 16 pieces and thus more fair. Only your opponent is using a magnetic chess board which he holds in his hands and starts running and jumping around with it. As it is Blitz chess and you only have a certain time to make your move, sometimes you simply don't manage to run and jump after him fast enough, so he makes far more moves than you do, and wins again.
You see some other players arriving, and with your first opponent being so obnoxious, you decide to play against them. Only they are three and play one move each before you can make one move. Three against one, you lose again. You retreat into a corner where you see a single lone player, and offer to play him. Only it turns out the guy is grandmaster Garry Kasparov, who not only beats you easily, but then proceeds to spit on you and call you a n00b.
You flee the scene, but on the way out get ambushed by a rogue chess player in hiding, who manages to make his first 10 moves before you can even make your first, then stuns you and makes another 8 moves, setting you check mate before you even moved your first piece. You finally make it out the door and decide to never play chess against real humans again, playing against a computer is obviously much better.
This, in a nutshell, is PvP in most MMORPGs. You get all sorts of ganking, stealth attacks, stun locks, or opponents running and jumping around in a way which would be totally ineffective in PvE. And unless there are special rules, like in arenas and battlegrounds, you will only ever be attacked by people who are of a higher level, or more numerous than your group. You play PvP to play against an intelligent opponent, but the first thing his intelligence tells you is not to fight you unless he is 100% absolute certain to win.
The reason why my chess story is so silly is that chess in reality has very strict rules. You're not allowed to jump around with the board, play three against one, or make more moves than your opponent. Chess is fair and fun PvP, because it has heavy restrictions. And we are far from getting those kind of restrictions in a MMORPG. The most balanced PvP you can have for the moment in any MMORPG is playing on a PvE server in World of Warcraft, where little or no overland PvP happens, and the action is limited to the better (but still not perfectly) balanced battlegrounds and arenas. Warhammer Online takes that situation and moves one step forward, two steps back. The step forward is as far as we know not having a raid endgame where you can improve you character further with epics, so that at the level cap all players are more or less equally well equipped. The steps backward are deliberate changes to make unbalanced open world RvR count for more victory points than balanced scenario (battleground) PvP.
PvP in MMORPGs *could* be fun. But for that it would have to be more like chess: With strict rules making sure every single game that counts is balanced. With leagues, ladders, ELO scores, sorting players by ability, and making them fight other players of similar abilities instead of ganking lower levels. And I don't think this is possible in open world PvP or RvR.