As far as the perfect MMO formula goes, I'm not entirely sure even Blizzard knows what made WoW as big as it was.
The question for me, from the moment it was announced and BioWare started talking about adding the story pillar to the MMO genre, was just how successful that approach would be. Because that's something that many of the MMO gamers I know have actually looked at as a negative. Even me to a point. Most of my time talking to NPCs in WoW, and in LotRO now, has been tabbing as quickly as I can through tedious walls of text to get to the next monotonous grind the game has to offer. And in that light I'm not sure that, at the end of the day, your average WoW player gives two shits about story. MMO's, so far, have basically been about giving players a never-ending supply of braindead clicking, and not about following a storyline or doing anything else that requires thinking.
I hope it works, though. It would be nice to have a little more meat in the genre.