As someone who spent quite some time with MMORPGs, I think a huge problem is the neccessary time investment to get a lot of the "more interesting stuff" going. In other words: A MMORPG usually is equivalent to the epic pen&paper campaign that can take your group years to finish. You don't want to do that as a fun event. What you want is the quick "one shot" equivalent, a kind of MMORPG which you can "finish" in an evening.

It's just a comparison. Obviously you're not going to do a pen&paper RPG with a group of 20 to 500 people (we did 20 once, it was... uhm... interesting?).