I don't think that Zelda U is going to be a tougher game than what we have had recently. I didn't find SS hard at all and i found its puzzles almost insultingly easy. the biggest thing for Zelda WiiU is its aesthetic, to me Zelda Wii U will play much more like OoT3D with the game pad being the HUD with maps, and items and other subscreens leaving the main screen relatively empty, from there the HD graphics will shine through uncluttered. As far as the game itself being the first Zelda game on the game pad, much like SS being the first game with motion control, i doubt nintendo would make Zelda U very difficult as to not alienate new players and all the common excuses that are made along with the game being an introduction to a new mode of control (which to me can only be made once if you're planning to knock out games annually or something close to it, where it can't be used is if, like nintendo so often do, only release at most two games on one console.)
So yeah, Zelda WiiU, to me wont be hard at all and im thinking that the visuals are going to be what nintendo concentrate on this time around more than anything else.