Guild Wars is the MMO that's not really an MMO. It eliminates some of the bad aspects of MMO's, and still has a good pvp system and a large world to explore and things to do that'll keep you loggin in. It also is rare in that it's an MMO with a coherent central story told through missions, and you can BEAT THE GAME. You kill a last boss, get a celebration, congratulations you won, you finished the story, here's some fireworks.
MMO's with monthly fees have a vested interest in rope a doping you along, never finishing the story to keep you paying and grinding.
Guild Wars also nullifies the pains of spawn camping for quests, griefing via open world pvp/corpse camping, it's death penalty is reversable without forcing you to grind gold, swift traveling doesn't force you to go afk during a boring ride/flight from area to area, and forced grinding is kept to a minimum thanks to low level cap (there is optional grinding for titles/faction points, gold for better LOOKING gear, etc if you want to do it).
It really is a genius game. Now, why aren't we all still playing it around the clock like other MMO's? Because like I said earlier, you can beat it. When you beat a game, there's some closure, it's satisfying, and you can retire it until you feel like replaying it. MMO's never give you that, you just keep playing and playing and they keep stringing you along to wring $15 a month out of your credit card.
A single player game, or console game or Guild Wars is like a movie, you buy it, you watch it, see a story from start to finish, finish the movie, and you don't replay the movie every single day, that movie was a good movie but you finished it. We are now free to watch a different movie. MMO's like WoW are more like a soap opera. Day to day drama, that never ends.. if you're LUCKY enough to have a central, coherent storyline (which WoW pretty much lacks, it's more of a clusterfuck than anything), it never ends, because the devs have reason NOT to have it end, to keep you playing and paying.