It's starting to become pretty clear - there really does seem to be a limit on the variety of MMOs that one can play.
Not on the number of them, but on the genres themselves!
First, we have the anime MMOs - from the cartoony look of GhostX to the more "realistic" look of Megaten or Aika Online - it's the same group of apocalyptic threats future, past or present, being solved by Our Teenage Heropersons.
Second, we have the disturbingly large group of MMOs that are "I Can't Believe It's Not World of Warcraft!" Allods Online is one of this group, so are Gates of Andaron and Knight Online. These are generally medieval village-and-forest games, With Magic For Great Justice And More Boring Humans.
Third, we have Holy Crap, I'm In Ancient China archetype, Loong, Dynasty Warriors, World of Kung Fu, and a host of others, all set in an incredibly well-animated Holy Crap I'm In Ancient China With Magic For Great Justice world, frequently with amazing riding animals and flashy spells, martial arts, weapons and armor and so on.
Fourth, you have games that are dominantly user-created content, such as Face of Mankind or Second Life, two very different experiences indeed, but both are dominated by people building and selling and working the universe - EvE Online is this, writ large, AND you have to pay to get in on the action. That seems a pretty good idea on paper, but the reality is, on the Internet, especially in games, or even in social simulators like the aforementioned Second Life - Beware the Trolls.
Sometimes, you will see a company take a chance, and go somewhere new, the way City of Heroes and later Champions Online did: into the world of superheroes; or the way Sword 2 did, by making a fantasy MMO that was completely gender-positive and set in a world far ahead of steel armor and warhorses. A fantasy that has magic AND guns AND both genders being completely playable!