I think the issue in MMO development right now are all the different benchmarks that have to be met so your game has a shot - In no particular order...
1.) Must have better graphics than WoW + still be able to run on most systems (this includes things like highly polished animations and so forth) - Better graphics are one of the main appeals to get people to switch games, but its troublesome to walk the tightrope of improved graphics versus being able to run on most systems.
2.) Must have enough content to keep both the casual player and semi-hardcore player entertained - Having content similar to WoW at its first launch is not enough
3.) Needs to be free of most major bugs and lag issues at release - first impressions are everything.
4.) In general the game can't just be as good as WoW or slightly better, it must be significantly better to give people motivation to switch.
The issue with all of this, is that creating an MMO that can fulfill all of these goals takes a lot of time. I believe WoW itself was in development for at least 5-6 years before its initial release, and even WoW had many flaws in the beginning that would be fatal to an MMO trying to match it now.
All that being said, creating an MMO that can grab significant market share is not impossible, but rather it's going to take a uniquely talented dev team, and (perhaps more importantly), financial investors who understand that rushing the game to market will not give them a bigger return on their investment.