I have confidence in Zelda U's success (still haven't met a Zelda game I didn't like), but I suppose there are a few concerns I have.
#1: A "unique" art style we've already seen.
An example would be using The Wind Waker's cel-shaded graphics. We already have The Wind Waker HD, so I dearly hope they don't do something like that. I doubt they will, though.
#2: Altering Zelda way too much just for the heck of it.
Aonuma's mentioned that he's getting tired of certain conventions in Zelda and plans to change the way Zelda is played in big ways. Hopefully he doesn't take this into mid-life-crisis territory, like turning Zelda into a level-select screen, text adventure, or something completely out of left field just to be different. Again, though, I doubt this will happen.
#3: A 2D game.
As I've said before, I like 2D Zelda. Still, we're getting A Link Between Worlds for that, and the Wii U is a golden opportunity to flesh out 3D Zelda.
#4: Overbearing gimmicks.
Historically, I like Zelda's "gimmicks," but I also see where they can go wrong (let's face it, Skyward Sword's motion controls were a big turnoff for a lot of people). I suspect I'd still enjoy a "gimmicky" Zelda game, but I can't deny I might occasionally be a little peeved when I want a more simplified gameplay experience and yet I have to turn my bedroom into a jungle gym to get the full experience.
#5: Lack of creativity/lame story.
These two kind of go hand-in-hand. Zelda's notorious for not always closing up its plot threads or holes, and I'm really getting tired of that. Let's redouble the efforts of crafting an engaging, detailed Zelda story that doesn't
abandon half its threads, eh? I'd also like to see less recycling of old Zelda nuances and more brand new content. Do we really need another Queen Gohma boss ripoff or an obligatory hookshot? I'm all for keeping items like bombs and the bow - that stuff is fun - but reusing "special" Zelda items just 'cause seems kind of forced and lazy to me.
It will be nonlinear, thus killing the story
Aaaaand once again I feel obligated to debate this. Stories can be interesting and fun without being linear.
Puzzles can be fun, devious, and varied without relying on multiple items obtained previously. Zelda's done it before, other games have done it before, we shouldn't worry about it.
Items can also be fun, devious, and varied without being obtained in a linear order. If anything, I see non-linearity as an opening point to explore item usage in unforeseen ways.
and taking us back to the dark ages.
Not necessarily a bad thing if those dark ages were awesome.
The thing is, non-linearity is not superior or inferior to linearity; it's just different. It's like calling apples clearly superior to ham steak or blue clearly superior to red. Both have their pros and cons. You may not prefer non-linearity and hope Zelda U doesn't utilize it, which is fine, but it seems a little extreme to claim it's inherently a bad thing for everyone.