I think to compare any game to another is a gross simplification unless it is literally a mere mod from another. (Such as the Desert Storm Mod for BF1942 that became Battlefield) Even then I have enough faith in gamesas and Mastheads to create their own unique game. Also, it is in their best interests to diverge from it quite a bit or people would not buy. So some how between their pride/self-respect and the business angle, they won't do it.
Of course, it being an MMORPG, they're going to have similar concepts and some WoW aspects were good, some were bad. I think I'll be happy as there will be a Fallout MMORPG, its non-stop interactive Fallout! It'd be pretty hard to mess up, especially with such a picky hardcoe fan base and original developers (one of them at least, I think?) you're just bound for an acceptable level of game. Of course, if they only manage to produce a less than sufficient amount, they can always expand. This is definitly possible if they end up requiring a subscription fee which will end up turning it into a cash cow therefore lots of potential for it to expand. Also, keep in mind that the Fallout universe is vast and generally fun so expansion is always up for grabs. If you're going to grow concern, grow concern they don't manage to make it. But since they have been fairly interactive through ALPHA stages (of all stages), it'll likely be fine.