The most impressive technology in the Wii U will be the controller, simple as that. There's a lot of stuff going into it, and that 'stuff' costs money to R&D and manufacture. It should be a pretty impressive piece of technology, especially for the wireless video technology.
It's not really surprising this is where Nintendo's bigger investment lies. It's the staple of the Wii U much in the same way motion/pointer controls were the staple of the Wii. It's not just a controller. It's part of the Wii U hardware package, part of the philosophy, and thus part of all those years of technology R&D and now shipping.
As for the actual processing technology, I'm currently sitting on "expect the worst, hope for the best". Yes, it should outclass current generation consoles, even if such evidence isn't apparent right now. Yes, it might take some time for a developer to flex the Wii U's muscles. Whether or not that difference will be readily noticeable is impossible to say, but the potential is there.
I do feel it will be heavily outclassed by Sony and Microsoft's next generation console, to the point where games look noticeably better. I don't really have anything to back that up other than fragmented pieces of information on all three systems, a vague assessment, and a dash of cynicism. I do firmly believe people thinking the Wii U will manage to keep up admirably with the next generation of consoles will end up disappointed. I don't want to say it's the Wii all over again, and I don't believe it is, but the "PlayStation 2 of next generation" in terms of hardware doesn't sit well with me and I don't agree with it.
End of the day, I believe Nintendo's R&D and philosophy with the Wii U, both in terms of spending and manufacturing, is producing a system that is compact, low voltage, low heat, and thus very stable on the processing front. In addition to this, they've poured quite a bit of tech into the GamePad, which is a major factor of the system, especially the impressive wireless video feed. Both of these points will noticeably impact the raw processing capabilities.
And that's my 2c for now.