Other than Watch Dogs, none of the games shown actually pushed the boundaries of what we've seen in terms of scope of a game. Eventually, all the technical achievements in a game's graphics go away if you don't create new situations with the technology. It was kind of dumb that they showed off a racing game, it's the genre you get bored of the most quickly as far as visuals.
I do think games like Alan Wake or even Forza 3/4 would be impossible on xbox, as you'd have to give up too much of the effects or physics that are important to the gameplay. And there are other games like Mass Effect or Red Dead Redemption that would take a massive dive in their presentation if you used last-gen tech. But for the WiiU, there's nothing on the horizon on either the next-gen consoles or PC that looks impossible on WiiU if you make it ugly enough.
