The mechanics of a language are often what scare people away from it—those repetitive grammar exercises, dusty old textbooks, the like. While playing MMOs might not teach the nitty gritty of the language, they certainly provide a different approach.
One of the first big hurdles you'll have playing a French MMO is navigating even simple menus and commands. The influx of new vocabulary may prove frustrating at first, but once you get a hold of it, you'll be treating it as everyday—which is halfway to learning a language.
Of course, it's not just learning a few new words. MMOs are massively multiplayer, meaning that there are a lot of other players... and if you're playing French, chances are that if you need to interact, you'll need to learn a few words. This functions as a sort of virtual immersion, safe at your desktop at home, yet still working in an immersive environment with the conversational, slangy, idiomatic side of the language. Granted, French gamerspeak may not be the same sort of thing you'll hear at La Sorbonne, but culture is culture and language is language—this is just another facet of it all.
Online dictionaries are a must, especially for the rapid-fire world of MMOs, for both translating all these bewildering French phrases being thrown at you and replying in kind. I would personally recommend the French portion of WordReference.com.
While it might be a little frustrating at first, it's well worth it. The influx of new vocabulary is far different from what you'd be getting out of a classroom environment. Playing a fun game also proves to be a fun and rewarding change of pace for either working on language skills or even just keeping up with old ones.