Resolution. Nintendo announced that the games will be the same as on wii but THEY ARE NOT.
Some neogaf members posted confirmation videos of upscale to 1080p. I tested it tonight with Twilight Princess. I put it in on the Wii. Uglier than I remember, but great considering it was actually a gamecube game.
I then put it in the Wii U. Noticed the difference at the first warning screen. Then really noticed it at the Wii menu on Wii U in Wii mode. Much clearer, brighter, and more colorful. Hopped into the game and the difference is quite noticeable. I don't know if drastic is the right word, but you can definitely tell. I checked the input signal and sure enough it's 1080p. On a side note I'm trying to see if my TV has an antialiasing feature to clean up those edges. So far I haven't had much luck as cleaning up edges is making things more blurry. Looks better when left alone....so far, but the game definitely looks better in 1080 on Wii U. Much clearer, cleaner picture. And I like it a lot. So Wii games on Wii U for me.
Started as a gamecube game. Got delayed so it could also be released on Wii. It was released on both systems at the same time which was with the launch of Wii. It looked good, but could have looked better. They didn't do a whole lot with the conversion to wii. Made it 480p and widescreen, and added motion controls, but little else.