Massively multiplayer games are really interesting. Persistent worlds, complex social experiences, lots of interesting design and programming challenges.
The question is how many games the market will sustain. So far there have been a few smash hits: Everquest and Ultima Online of course, but also Dark Age of Caemlot and maybe Star Wars Galaxies. This graph of subscriber stats suggests a bunch of new games are taking off without really cannibalizing existing populations.
As seen on Greg Costikyan's blog