The idea that you're going to have a "completely different game" depending on which race/class you choose is a nice thought, but is going to be very hard to implement/keep up with.

Think about the content in TBC and WotLK for WoW. Sure, there are horde- and alliance-specific quests and items, but the vast majority of the content is shared. That's for good reason. Designing quality content is hard and takes a long time. Saying you're going to exclude people from half (or more) of the content because of the class you chose means you have to do two (or more) times as much work to keep them satisfied.

Given the amount of time it takes to get a decent WoW expansion out the door, I question whether this is going to be a feasible long term strategy.

I don't think it's impossible, but the dev tools and game engine are going to have to be absolutely stellar to enable the developers to keep up with the demand for new content across the board.

I think it can be a great game even if they can't follow through on this concept, but I'd hate to see this game get hyped up like WAR did, then wind up promising way too much and having to cut back significantly. I, like many other folks, am looking for a game to tear me away from WoW -- it's an awesome game, still, but I've been playing it since day 1 and I'm ready for something else -- but that game is going to have to be almost flawlessly executed. I'm scared that this concept might get them off to a bad start.