I am indeed curious as to whether Mario 3D World was their original plan or not. It is the kind of game they could have made quickly enough to be a reaction to 3D Land's surprisingly great (beat NSMB2) sales.
The root of the gamepad's appeal is for off-screen play. It's targeted, really, at non-american markets, where folks are more likely to live in smaller dwellings and maybe have only one good TV. Asymmetrical gameplay is just a side idea.
Whatever the concept though, it certainly doesn't seem to have any pull on the market at all. Nintendo just needs to start focusing on games, and what they have shown so far is a good start: it's not like GameCube where every major game is experimental in some ways (if anything, 3D World is too unambitious, but if it does for Wii U what 3D Land did for 3DS, it will do its job neatly enough), same with Donkey Kong, where we're still on the fundamentals. Mario Kart looks to be a stand-up successor to the vastly successful Mario Kart Wii (moreso than 7, where the rush of panic mode was felt in the paucity of karts and playable characters). The games from Platinum and Monolith soft are good fodder for the fanboys, Smash Bros is Smash Bros (quite excited to see more there)
How the Wii U ends up, and how it is viewed in posterity, will be an interesting thing to watch.