I think the difference with the Wii U will be that its gimmick will be harmless. I'm saying that for 2 reasons:
1) The touchscreen isn't something developers are forced to make use of, and if they want to make use of it they can just use it for a map or HUD or something easy and simple like that. Its not going to be like the Wiimote where a game has to be radically altered in order to function with it. The Tablet controller can simply be a traditional controller, and the touchscreen doesn't have to be employed at all.
2) It looks like the competition is going to rip off the tablet idea. This is bad for Nintendo having yet another of their innovations stolen, but its a good thing for developers because it makes their task of making multiplatform games much easier, because all three consoles will have similar controls. Again, that's very different than how it was with the Wiimote versus the traditional controllers of the competition.
So the Wii's gimmick ended up being a bad thing for developers, but the Wii U's gimmick is at worst something that can just be ignored. The Wii U's gimmick is pretty much just a console based rehash of the DS' touch screen gimmick, and I don't need to point out how extremely successful the DS has been as a system, so I'm certain the Wii U will do well if not great as a console.