Virtual Worlds are Works of Art
It's easy to see that virtual worlds require a certain amount of artistic craft to pull off. The game setting for World of Warcraft, for example, involved incredible set design, animation, and texturing. A good campaign setting requires some good writing and creativity to pull off. But that's not the same thing as saying that a virtual world is a work of art.
Consider the world of Middle Earth. While the novels themselves comprise a great artwork in their own right, does the world deserve to be considered as art in its own right? If we leave aside for the moment the fact that the world can't be separated from the works, we could admit that Middle Earth has the power to inspire, that it has nuances, that there are compelling ideas there which we can discuss and debate.
The problem is, with a novel, you don't have full separation of the world from it's development into a particular story. But with a virtual world, you can.
A great virtual world is potentially beautiful, nuanced, and thought provoking. Like a painting or text it has static elements that can be considered in isolation. But like a movie or a drama, it can also have dynamic elements that simulate an action.
I think that the artistic possibilities of virtual worlds have barely been touched. If virtual worlds are works of art (and I emphatically beleive they are), then it should be possible to create virtual worlds that aren't only great fun to play, but stimulate the mind and the soul.