This article shows exactly why Runescape is the only other MMO in the with over 1 million subscribers in the US, it's simply designed and quick to pick up. The problem is that it's too simple though, and there's not much beneath the surface.

I disagree with the above comment and idea that simple and "easy to jump into" design can be too accessible. True, any successful game is going to come with riff raff, but games should be quick to pick up and at the same time difficult to master. I feel like WoW does a good job of this and that's why so many people play it. I don't think a game qualifies as "hardcore" just because it takes a 4 hour tutorial to figure out the poorly designed interface and game mechanics. That's just bad game design.