This is a simplistic analogy but my own expectation for how Wii U will seem better than this gen, in the long run, is like comparing 3DS games to Wii games.
3DS is technologically more advanced than the Wii in a lot of areas, such as its shader support. However, in terms of outright raw power, it is weaker in some ways.
As a result, some 3DS games, mostly early ones, are slightly less advanced that top Wii games like Mario Galaxy. But in many ways, most 3DS games look much nicer than any Wii game.
Over time, I am expecting most Wii U games will not exceed the finest performance of the PS360 when measured by a few criteria. But, especially in games built for the console from the ground up, the stronger GPU will end up making a lot of things look much nicer than the best PS360 titles. And the increase in ram will greatly improve average texture quality, promoting more visual consistency. Also help with loading schemes.
In short it comes back to what I had originally theorized - Wii U could be seen as this generation's HD consoles but optimized, with their bottlenecks and inadequacies removed. Such as chugging framerates when the pretties are turned on, lack of AA in too many games, wildly inconsistent texture quality sometimes even within the same game. And many games not hitting 720p.