I personally prefer Guild Wars. At first, I thought being alone with just you and your party once you go out in the world would be terrible. No kill stealing seemed like a good thing, but I was one of those kill stealers, I admit, I was a terrible person would run up at the last minute and completely destroy whatever someone was killing, just to take all the xp and items.
It was not a problem for me. It was not until I had played the game for months that I realized, I don't even care that I'm out alone. Guild Wars story(s) were solid enough to draw me in, make me feel as if I were playing a single player game, with optional co-op. If I wanted to play an MMO, bring people, if not, leave them in town. Then there was the PvP. Another thing I though I would be disappointed in. It seemed relatively small scale. In a simple 4v4 match, the fun is endless.
One point that annoys me, yet I praise Guild Wars for, is the limit on how many skills you can have at once. By only allowing you to take eight skills into battle, they limit the powers of each individual player, but they greatly increase the challenge.
The Level 20 cap, another thing that I thought would bother me, kept things interesting. When I hit level 20 and stopped getting stronger, the monsters I was fighting didn't. They kept getting stronger as I went, adding a new challenge to the game. I couldn't just leave and grind until I was stronger than the enemy, I had to plan, make myself a better player, not the same player with a stronger character. By capping out the level at 20 they brought strategy to a whole new level. By limiting you, through the level cap, the size of the party, the 8 skills, they make the game more interesting.
Though it may just be because I like a challenge, the harder something is, the more I like it.