I think that this game was developed under the impression that WoW doesn't ever change.
The Burning Crusade changed so much about the original game, and the new expansion is set to change even more. Sure, the basic mechanics are still the same; fire mages still cast Fireball in end-game raiding like they did before, things like that. But those basics are the foundation, and everything else changes with a new WoW expansion. It's not just a few new places to go kill stuff - or, it isn't for anyone who plays more than 10-12 hours a week.
I imagine that Warhammer will be moderately successful and will likely do better than Conan did, but it's all about what they can do to keep people at the end-game interested. Conan didn't do much at that level at all, even with all of their PvP action - but that's mostly because Conan's PvP was fairly poorly designed and still overall an afterthought rather than a basis for the game.