Personally for me, wow is hands down the greatest MMO to ship, and I've worked on many and played most. Best world, best story, best look and feel, best gameplay, best raiding, best time versus reward balance, best class balance.
I point that out, not because I would ever wish to have a subjective discussion of value at this high a level, but only to provide balance to your thought process when you think about who and what type of player you are targetting. Inability to parse out why wow works for so many on so many levels, regardless of what you personally look for in a game, is a handicap that I don't want any of our developers to have.