All I look for in an MMO is PvP. However, I've realized that most people want a lot more than that out of an MMO. For example, I was having a lot of fun playing AoC and probably would have kept playing except everyone else was quitting and I'm not a big fan of empty world MMOs. If there's one thing that kills an MMO faster than lack of good PvP it's lack of crowded zones in all level ranges in my opinion.
I'm worried this game will launch without a lot of dungeons, raid content, player cities, etc to keep the average MMO player entertained and therefore will end up being a huge empty world full of PvP people with no one to attack but each other once every few hours when they see someone. All I've heard about in beta is people killing some mobs, getting a bunch of gear, then fighting players. Not a single world on dungeons, raids, cities, anything beyond the most basic things.
I'll be really surprised if it doesn't pull an AoC and become popular for a month then die when everyone realizes there's nothing to do after you've killed someone for the millionth time.
Good PvP is fun but it's the PvE content to fight over that keeps it interesting. There better be raid mobs and dungeons where you bring your group of friends and beat the shit out of some people and take over their camp. That is the kind of thing that makes PvP worth doing.