I'm less interested in gameplay mechanics than the lore they intend to introduce. Unless it's going to be non-canon, which I doubt, a Fallout MMO is going to have to introduce a fairly large amount of new backstory to the world. If it's just a WoW clone, then as far as I'm concerned, fair enough. I've heard WoW is supposed to be fairly popular anyway, so they must be doing something right. It's more about the world to me.
If it's just travel-to-plot-of-arbitrary-wasteland-A-and-shoot-radscorpians, then travel-to-plot-of-arbitrary-wasteland-B-and-shoot-raiders and then travel-to-plot-of-arbitrary-wasteland-C-and-shoot-mutants, then they might as well have not bothered. Also, there will be quite a lot of people running about, so there really needs to be some way of explaining that whilst keeping with the feeling of world in which survival is key, and not always guaranteed.
Whether or not they can make it more or less like WoW is completely irrelevant to me, and wouldn't be even if I did play WoW. But if it doesn't feel like Fallout, then the whole uproar over who's actually going to make is will have been completely for nothing.