I think Twilight Princess is clearly the product of troubled development, but I don't really find many of the criticisms very valid. I highly doubt anyone who found Twilight Princess "empty" is going to enjoy an open-world Zelda, because there hasn't been a single 3D Zelda that's had every inch of the overworld filled with content, nor are there many open-world games that are like that either. I mean, I haven't played every open world game, but SotC? Every Rockstar game? The Wind Waker? Those are all games with wide spaces of nothing in between meaningful content, especially in the case of the latter where the scenery doesn't even really change most of the time. I hope Zelda U spaces things out in such a way that there are no corners of the map in which there's nothing at all, but I don't think it's fair to expect every inch of the overworld to have something major to do when even the more compact 3D Zeldas don't have that, and no open world games that I know of do either.
It's a matter of opinion, of course.
Personally, I did find WW and TP a little empty, but not necessarily because there wasn't enough to do. For me, "rewards" for exploring don't necessarily have to be gameplay. I'd
prefer gameplay in the form of sidequests, bonus dungeons, and lots of little secrets, but that's not the only option. Art can count as a reward for me. If I go exploring and find a hidden viewpoint that offers a noteworthy view of a beautiful, well-designed game world, that can be enough. Xenoblade, for example, has several of these, and the only "tangible" rewards you get for finding them are a few experience and skill points. I still had lots of fun finding them because the environments were artfully and creatively designed. I'll call it empty if there's too little gameplay hidden in places like this, but I don't need it in every single corner.
So what I said about WW and TP earlier applies not because their worlds didn't have secrets and things hidden around them, but because their worlds were
flat. WW's optional islands were tiny and uninteresting outside of a heart here or there. TP's Hyrule Field was mostly a perfectly level expanse connected with very artificial corridors.
Worlds in video games are interesting because of how you interact with them. If they're nice enough to look at, an appreciation can be enough for some small segments. If they're not, the lack of gameplay is felt more harshly. TP had, where you exited Kakariko to the north, a large rocky cliff face made of several different levels and shapes, and it was scalable. There was a Piece of Heart at the end of it, but there didn't have to be. If there were more structures like that in the environment, just having those options for interaction would be enough, even if not all of them have a "reward" at the end. The fun of climbing and discovery can be its own reward, especially in a game like this where you can then jump off and drift down to rejoin the world safely.
Being "nice enough to look at" isn't a function of power, either, it's one of composition. Old pixellated games can still capture the imagination if there's enough art in the composition of the environment.
Some people don't find those things interesting and are only in it for the items at the end. They'll
usually (though I certainly can't speak for everyone) prefer games like Skyward Sword or the 2D Zeldas, where everything is much more compact and more puzzle-like. But I spent time jackknifing between Skyloft's two primary islands on my Loftwing because the view and the sense of speed were fun. If you want every nook and cranny to have secrets, Zelda Wii U will probably disappoint you, but I'm pretty excited about finally getting a 3D environment that looks crafted to be interesting on its own merit.