Compared to its excellent PvE part, the PvP in World of Warcraft is by many considered to be weaker. Which is the reason why many of the games under development that are hoping to become the successor to WoW are stressing better PvP as a selling point. But lets have a look what kind of PvP can actually attract large amount of players, and what kind of PvP is forever condemned to niche games that nobody plays.
The theory behind the attraction of PvP in a MMORPG is that computer-controlled opponents are stupid and act in repetitive ways. Fighting a human opponent is more fun, because the other player is more intelligent and unpredictable. Thus the situations in a PvP fight will always be different, which is the closest to what a MMORPG gets in player-created content. The big flaw in that theory is that humans are actually *too* intelligent for PvP. Given a choice, a player will initiate a PvP combat only if he is sure to win. Thus the attacking side in free-for-all PvP is nearly always either several attackers ganking one victim, or a higher level player ganking a lower level one, or a player abusing some other situation, like stabbing his victim in the back while the other guy is fighting a mob.
Ganking is the curse of PvP. It is extremely frustrating for the losing player, up to a point that players who get ganked repeatedly are quitting games. When UO was changed from free-for-all PvP to having a choice between PvP and non-PvP sides, this change was done to save the game, which had been bleeding subscribers due to ganking. And seen from this angle, World of Warcraft PvP is actually rather good, if (funnily enough) you play on a PvE server. In duels, arenas, and battlegrounds the numbers and levels on both sides are more or less evened out. Perfect balance is probably impossible, but WoW PvP is as balanced as it gets. The future of PvP is PvP that is restricted to fair fights.
We don't know enough yet about the upcoming PvP games, AoC, PotBS, WAR, to say how far PvP in them will be restricted to fair fights. There are some indications that WAR is trying to balance PvP out, it certainly isn't pure PvP. And for both WAR and PotBS it is known that there are PvP-free areas in the game, although we still need to see how viable it will be to play these games while avoiding the PvP zones. But one thing is clear: None of the announced games has really free-for-all PvP. So there is hope that in spite of the PvP-centric marketing these games will actually be playable for the large majority of players who don't like being ganked.