I don't play MMOs. It has nothing to do with the setting of most MMOs (they are most fantasy games, and I don't mind Fantasy), but it's the structure. MMOs are about loot, social connections, guilds, and stats and big mobs. I don't really care much for loot, I have no social life, I can't stand the thought of a guild and while I don't mind stat driven games, I can get that from RPGs.
Even if they make a Mass Effect MMO, I wouldn't play it, because it's an MMO and MMOs are not for me. They're GREAT for some people. Just not me. No matter how much I love the Mass Effect universe (the Massiverse? Effectiverse?), MMOs are just a bad way to tell a story. Seeing numbers fly all over the screen, fighting the same boss over and over again, seeing hundreds of ill-sized player characters bunny-hop across the screen whilst chat messages pop up don't immerse me in a game's universe. I'm one of those people who actually LIKE the fact that games have gotten more "Cinematic" in recent years. MMOs do too much to remind you: "You're in a game! A GAME! None of this matters! It's all a big sandbox for loot! YOU ARE PLAYING A GAME!!!!" And that just KILLS the fun for me.