I don't think it'll be a flop. I think it'll be a success and maybe even a critical success and a runner up to WoW in the United States but I just think it'll be a long, rough road before they fix all the annoying peoblems with the game right now.
I've been reading up on this game a lot and have waited for the game for many years. Most of the review have been positive with some skepticism about performance and mechanics. As long as the developers seem flexible and willing to learn from their mistakes I'm sure it'll do well a year from now.
Warhammer, on the other hand, seems like a WoW clone with many things about it that may even be worse than WoW. The only silver lining about it is the Realm v Realm aspect about it they does seem to be better than WoW form of purely instanced PvP. I think AoC will do better than Warhammer but I think both games will see a small migration of players from WoW.