An interview of Ralph Koster in RealMMO addresses some interesting question regarding MMORPG:

I think WoW sets us back only in specific ways. I think it moves us forward in other specific ways. Blizzard, as usual, nailed polish, nailed guiding the player, nailed a look and feel. They took the old formula and put it in really snazzy bottles. That’s what they do best, and they are very very good at it – the best in the industry. But they also didn’t pick up the ball and run with a lot of stuff that are growing trends in the MMO industry today – and what’s more, given their expertise, they probably never will. We’re seeing a lot of interest in stuff like user-created content, in-world economies in games like Eve Online, and so on, and we don’t see anything that sophisticated in WoW. WoW is very much a “theme park” sort of world, one which is about putting you on a ride and letting you experience it. (...) I hope the next big thing is what we’re working on! If I had to sum it all up in one world, it’s “choice.” One of the things about the more directed games is that they really don’t give you choice. “You pays your money, you takes yer ride.” You don’t get to hop off midway or try out different ways to play. And while pretty much everyone enjoys a theme park ride at least once, the number of people who come back to it over and over is relatively limited compared to the broader array of activities in the world. We lose sight of the fact that WoW is big, but MySpace dwarfs it.

Why do I blog this? because this interview of Koster starts scratching the surface of some avenues MMO that should be pertinent to explore. However, user-created content in an MMO can also be thought as very similar to the object creation in MUDs (the only differences here are that it's in 3D and that it can be sold).