Besides the fact that it's running on an emulator, you can clearly see a difference on the backgrounds and effects (like heat distortion), textures on the foreground too. And then there are some little things, like the coins and clouds which are much better.

Sure it doesn't look all that different but that's mainly down to style and Dolphin.

Yes, it's Dolphin, can't you people read, it says Wii HD emulation on video? Eurogamer did comparison to see how much U version is technically improved, and the only way to do that is to run Wii version on Dolphin in 720p (so I suggest watching it in that resolution) and compare it with U version.

The Dolphin emulator is very good otherwise they'd look a lot more different.

Even with that the Wii U version has better textures, backgrounds, and a lot of various improvements to things like water/liquid effects, certain enemies, lighting, etc.

Obviously there is a lot of shared resources that had little to no upgrades with the exception of better resolutions. Doesn't really matter as both games are fantastic and well worth the $$. (especially considering I got my copy of NSMBU for free)