Failure is relative. 415,000 subscribers to WoW's over 10,000,000 subscribers means that Age of Conan failed in my opinion which I'm VERY extremely happy about.
The numbers just don't backup that opinion. There's plenty of mmo's around that have far less players and are thriving. 415 000 Subscribers equals about 3% of the market and that's not bad for a game that just been released, considering there are games that have been around for much longer have a much smaller player base.

http://www.mmogchart.com/Chart7.html

The game maybe didn't live up to your expectations, but that doesn't make it a failure.

This further identifies what I've been saying on a lot of message boards about new games that come out. The more different they make the game from WoW, the more likely it will not do as good. Most people have a lot of different ideas about how their perfect MMORPG should work.
It's actually the other way around. The more you make a game the same as another game, the more people will feel that you're just copy of that other game. The market is currently already saturated with fantasy MMO games:

http://www.mmogchart.com/Chart8.html

With more being announced and about same number being canceled every month (quite often even before the game has been released). It's actually the mmo's that try to do something completely different that have the most chance of succeeding. This is because they create their own niche market and don't have to compete with all the other mmo's that are all trying to do the same thing.

My view is WoW is just about perfect as far as game play. There's only a few bugs here and there that are irritating but as far as the game play, WoW has it put together very well.
As a game developer, I've looked at thousands of different game mechanics, played/tested hundreds of different games to find out what does and doesn't work, to see and feel what the strong and weak points of those games are. In my experience most games have several features that they are doing better then their competition. WoW certainly has a higher number of good features then most other mmo's, but that doesn't make it perfect. It has its share of gameplay failures too.