I have played a lot of mmo's, as I am sure most of you have. I would have to say, customer service, in game GM's and in game events, to bring the community together to sti8r excitement. When problems are addressed in-game, and you are not give a cut-and-paste response to your problems, it makes a difference, human interaction. When GM's appear in game and start an event it revs up the server. From there could be graphics and content. Graphics is a must in any game worth it's weight. But the content needs to be such that you feel immersed, you need to be a part of what you are doing, not just a simple participant. From there I would say missions and pvp. I myself need to get a mission/quest or something that drives me, I need a goal, not something like mindless slaying.
Just my 2 cents :)
If you don't have time to do it right the first time, when are you going to find the time to do it over?