You know, if they try to make the game appeal to everyone, it'll probably end up appealing to no one. If it's not like normal RPGs, then the RPG crowd isn't going to like it. If it's not like normal adventure games, then the adventure crowd probably won't like it. If it's not like normal action games, then the action crowd probably won't like it.
Sure, it might fill a void, but it might also simply be a mish-mash of ideas that don't really come together except for the few that really play the game to death. Black and White didn't really work so well because you NEEDED to spend a ton of time training your stupid animal, and then it did what you wanted it to do anyway. Most people didn't care -- they wanted to play the game, not waste time training a cow to play the game for them. And the people who did figure the game out and spent time training their animal had a lot of fun... for the 5 available levels, only 4 of which are really actual levels. Everyone else avoided the game.
I'm hoping I'm surprised by Fable, as the few things I've heard sound really cool in practice but haven't shown up a lot in gameplay (except for people saying different things to you and treating you differently depending on your status -- they've been doing that for years, most notably recently in skies of arcadia). But it looks like it'll be a graphically pretty game that goes all over the place without having a real strength in any one area, making people bored with it, and the only thing giving it length would be playing around with the engine to see what you can do to it. That's more like beta-testing than playing a game, in my mind.