If a Sci-Fi themed mmo was available that was actually fun (Galaxies was ruined and had no feeling of the potential of the Star Wars galaxy) and had some actual role playing content other than just shooting at stuff then I would be all over it. What is so hard about creating adventure and story to a sci-fi themed world when you can have giant space stations missions, ship corridor battles, planetery missions with exotic space ports and cities and creatures and so on as well as being able to trick out your character as a bad ass on the edge of the law with a starship barely held together with spit and gum, while working towards putting on new engines, turrets, pods, modules etc. Robust trading and skill as well as the ever present playable evil mega space bad guys who control the galaxy. Come on Star Wars was all that and more, then what the hell happend? To answer you question, Serinity/Firefly mmo done right. Hell Ya!

I believe that the people are more interested in ancient history like settings. Swords, maces and knives and maybe even guns do great. Magic is a fantasy thing that people always liked ever since ancient times!
Like Todd said most settings which go beyond our own imagination (sci-fi etc) is just harder to get done right. You have to come up with a very interesting story that appeals to everyone.