Shamelessly stolen from Gonintendo.

"The Wii U eDRAM has a similar function as the eDRAM in the XBOX360. You put your GPU buffers there for fast access. On Wii U it is just much more available than on XBOX360, which means you can render faster because all of your buffers can reside in this very fast RAM. On Wii U the eDRAM is available to the GPU and CPU. So you can also use it very efficiently to speed up your application.

Theoretical RAM bandwidth in a system doesn’t tell you too much because GPU caching will hide a lot of this latency. Bandwidth is mostly an issue for the GPU if you make scattered reads around the memory. This is never a good idea for good performance.

I can’t detail the Wii U GPU but remember it’s a GPGPU. So you are lifted from most limits you had on previous consoles. I think that if you have problems making a great looking game on Wii U then it’s not a problem of the hardware."

- Manfred Linzner, Shin'en Multimedia

http://hdwarriors.com/why-the-wii-u-is-probably-more-capable-than-you-think-it-is/

http://www.gonintendo.com/?mode=viewstory&id=212746

tl;dr version: "if you cant make a good looking game on the WiiU, then you're just lazy."
that's directed at you, 3rd parties.

btw, i've played their games on the Wii/U, and they look incredible.

It is wild that, almost a year after the hardware releases, developers are not allowed to detail the GPU. There's a difference between Nintendo not trumpeting the system's specs and forbidding others to do so. What do they gain from that opacity?

... that is true for all current systems (3DS/Vita/360/PS3/WiiU) !
art style > technical prowess every single time.
(imo) Xenoblade/Mario Galaxy looks better than Crysis 3

Oh, they can make nice looking games. They just don't want to.

if you take a look at what they have done with the GPU and CPU inside the Wii U, they have a lot to hide from not only the public and publishers, but mostly from the other companies. Even though Wii U is backward compatible it doesn't have a separated Wii chip inside. The equivalent of it is embeded on the Wii U GPU itself allowing it to be used for both the Wii and Wii U games. Who knows what else does it have. Nintendo has spent too much money and time trying to do those kind of things that are different from any other architecture seen before that they just decided to keep it on the shadow. It is more a bussiness strategy than any other thing. Besides, developers can see all of those things by themselves by just talking their moment with the hardware, but of course they are thinking on making one game alone for the console, so they really don't care how the game performs. Also, they rarely deliver a game without need of patches. I am talking about big third party publishers. 

In Russia we not call it 'you're lazy' but 'your hands grow from your ass'.

I can understand what they're thinking.  If specs would be given, it would be much easier for people to trash the system.  Sure, not revealing specs makes people think it's lower than it is... but in the end it is all conjecture to the public.  So when a game looks better than what people think the hardware is capable of, the game SEEMS better.  Or so this line of logic can follow.