I would personally say a balance of story and gameplay for an MMO would be good.
FFXI did have good balance, and the leveling isn't mindless grinding like other typical MMO's. You actually have to fight in parties to gain experience points and every time you get in parties you learn how the system works, you learn how each job(samurai, white mage, paladin etc etc) plays a major role in parties.
The story is also pretty good for an MMO and the game is relatively difficult requiring you to form teams/parties when undertaking most of the quests and story missions.
I have a great feeling that the only game that is going to surpass this kind of quality is the old republic from what i have seen.
I have played many MMO's and many of them fail to actually impress me and keep me interested, FFXI was the only MMO ever to succeed in doing that.