Everything in an MMO is intertwined. The ether is filled with MMOs that have one decnet thing going for it. To be truely great, an MMO has to do several things well. So I could pick five, but one? Who cares if it gets one thing right. That'd be like picking one specific feature of a potential girlfriend--it's the package that matters.

Player lead plot - The ability for the MMO to be effected and changed by player choice. The GMs working closely with the major factions/alliances/guilds allowing for extra personalized details to being added. Having major player/entity stamps added to the world, adds more depth to the story and playing field. Maybe like having some guild halls for major guilds, say the top three in the city, while lesser guilds have to share.

I used to LARP and in the big events some of the story lines came from the factions and guilds, sometimes they where the major plot for the weekend, other times they played a minor role. It allowed for people to get involed in smaller storylines and follow them. This also reduces some of the strain on GM as the plot can be fowarded in other means. All player lead plot had to be passed by a lead GM before allowing to happen.