MMOCrunch tackled this topic in yesterday's blog entry. He listed several "key factors" that he feels make a good MMO.
The first point he touched on was some sort of in-game holiday event. Nearly every game has something to acknowledge the holidays, whichever ones you might celebrate, and Russell referred to them as a sort of "holiday thank you from the game developers." Another item on the list was one that we've seen come up time and again: good communication with your fellow players. Keeping in touch with the community helps to keep the game from becoming stale. These are just a few points touched on, so be sure to read through the full entry and see if you agree.
Again, there are as many opinions on what makes a game great as there are players out there, but this provides some good food for thought, and is one worth taking some time to read.