SPEAKING OF WHICH THIS IS INFORMED SPECULATION BASED ON WHAT I HAVE HEARD DEVELOPERS TALKING ABOUT LATELY
DO NOT TAKE THIS AS FACT AND DO NOT START CHICKEN LITTLEING BECAUSE OF THIS POST. EVEN IF EVERYTHING I HAVE HEARD IS 100% ACCURATE, THERE COULD BE REASONS IT WILL NOT HOLD TRUE FOREVER.
I'll say that you should not expect the moon with the Wii U.
Everything I have been told is "best of the current generation by a country mile but not significantly beyond that."
It is the XBox to the PS2. The Gamegear to the Gameboy. The Mad Men Alison Brie to Community's Alison Brie. It'll look better, but generational transition probably won't apply to it.
On the plus side, there will probably be nothing hardware-wise stopping downgraded ports from what people think the new XBox will offer. We're talking, like, Witcher 2 on high settings vs. Witcher 2 on low settings. Same game, same basic idea, different graphics.
But games that do take advantage of the Wii U's hardware are going to blow this current gen out of the water. I am not sure we'll see that at E3 (seriously, think back to nearly every console's launch, most of those titles don't stand up today), but I'm sure we'll see it eventually.