Well by caster, I mean something like a mage or wizard. Something like throwing fireballs/frostbolts/etc (doesn't have to be those, just similar).
I guess it doesn't have to be fantasy, but to be honest I don't have very strong interests in non-fantasy MMOs. Something that bridges futuristic and fantasy might be cool, though; Too Human was an interesting single-player game that did this.
Aside from that, I feel like fantasy MMOs are better for those who enjoy exploration of a vast, dangerous world. Lack of mastery over the entire planet with technology and such enhances the feel. The added travel time make the world feel more "real" and large as opposed to feeling... well, like a game. When you can zip from one end of the continent to the other in a couple minutes it cheapens the experience somehow.