I think there are a couple of different ways the game could go, but I don't think the game will "bad" even in a worst case scenario.
Best Case
The transition to open world is seamless and natural - Zelda U implements an open world that doesn't try to compromise quality in the name of quantity, and the world that we're given is both interesting and rewarding to explore. The side content in general is fun and provides some interesting stories on its own, enough to make seeking such content worth taking a break from the main story. At the same time, Zelda U doesn't put such an emphasis on exploration that dungeon design suffers in the process. Dungeons are varied, provide interesting mechanics, and maintain a great atmosphere that makes each one stand out. The game uses the gamepad/whatever other input device well, and the game handles perfectly.
The game doesn't allow the open world to get in the way of telling a decent story, and the story ends up being quite nice with some great moments. To do this, the game has a strong cast of side characters, a memorable villain, and a companion who manages to stand out as a great character without any of the hand-holding flaws of past companions. All in all, in the best case scenario Zelda U would bring together the individual strengths of past entries in the franchise without taking their issues as well. In this case Zelda U would likely be the "definitive" title in the franchise, but the chances of the game putting together so many series strengths without having its own issues are little to none - its why every other entry has had its own strengths and weaknesses.
Worst Case
In the worst case scenario, Zelda U is clearly a cut below past entries in the series. It takes too many risks in the process of trying to be some kind of "revolution" on the Zelda formula, and in the process none of the risks turn out well. Exploration ends up being a case of content quantity over content quality, and as a result most of the things available to do feel like things you've done elsewhere in the world - and they weren't too fun the first time around either. The game also sacrifices in the dungeon department in order to implement the open world, delivering uninspired dungeons that just come off as completely mediocre. The core gameplay of Zelda U isn't great - its still probably a step up over most games out there, but its a far cry from the other entries in the series.
Based off some of the trends with other Nintendo franchises lately, the worst case scenario for the story would probably involve either a massive amount of hand holding or simply killing off any real story. In either case we'd probably end up with a story that simply has no moments worth mentioning, and leaves no real reason to care for anything in the world. Characters would be unremarkable, and the companion would feel more like a talking game manual than anything else. This scenario also seems a little extreme to me. I doubt that we'll see a 3D Zelda game as weak as the one outlined in this scenario, the chances of it happening are about as likely as the near perfect game outlined above.
Most Likely Scenario
I thought I'd go ahead and outline what seems most likely to me, since the best/worst case scenarios seem rather unlikely. In truth, I'm expecting Zelda U to be to the same level as every other 3D entry to date - some elements of it are phenomenal, while others are below the standards set by predecessors. With Zelda U I'd probably expect it to be something where the core exploration is actually really enjoyable and rewarding. You'll probably hear people going on about it for months after the games release, and it'll be one of those things that keeps the game in good standing in the franchise. On the other hand, the game will have some flaws like the other entries - unremarkable dungeons, a bunch of annoying side characters, something along those lines. This will annoy plenty of people, and it will cause some to hold the game in poor regard relative to the other games in the series.
The end result in this case would be roughly the same result from every other 3D Zelda game: a game that's generally well received, but not some kind of definitive "greatest Zelda game ever made." Like Majora's Mask, Wind Waker, Twilight Princess, and even Skyward Sword, Zelda U would have plenty of people who hold the game to the highest regard in the series while others simply think it does everything wrong. I think that's the most likely scenario, simply because that's what we've seen happen with every past entry.
