What keeps players coming back to an MMOG.
I actively dislike MMOs and have limited experience playing EVE, that Warhammer Fantasy one, Secret World, and MechWarrior Online. I can only tell you what I found compelling and liked about a few of them and what made me run from all of them.
Things that Made Me Want to Stay: An art style that is tonally resonant with the story. Good UI design that allows me loads of information when I need it but doesn't clutter the screen when I don't.
Things that Made Me Want to Holler: The BUY! BUY! BUY! mentality of the item/whatever stores -- I don't like spending real money on imaginary shit. Obvious grindy game mechanisms. Characters that MUST progress in a particular way to remain effective.
But, like online forums, it was generally the people who drove me away.
May I suggest that, if my acronym memory serves, you drop an M? Lose "massively" and you'll likely create an fun multiplayer game.
I'm just an old man lamenting the days of LAN parties...
Good Luck!