When the Wii was released, me and my brothers -although huge Nintendo fans for years- were skeptical. The new controller interface was either going to be revolutionary, or completely flop -or at least that's how we saw it. This was a big step for gaming, and a huge risk for Nintendo. Looking back on it, I would still say that it was a huge risk, though I'm not exactly sure if it has paid off. I would say, however, that it was neither revolutionary, nor a flop: it sat somewhere in between, which I was not expecting at all.
That being said, many people would classify the Wii as a "disappointment", even though it did very well in terms of sales. For a while, I wondered why this was, but as the Wii U is drawing closer to release, I think I've come to a speculation. One of the biggest aims for the Wii was to get everybody playing video games: your grandma, your kids, everyone. It's focus wasn't really to keep the focus of the gamers that already love and play Nintendo consoles. You might disagree; after all, Nintendo is still creating Mario games, so shouldn't the older gamers still want to stick around for that sake of nostalgia?
That's partially true, but an issue that is far more dramatic is how much (a) those gamers have changed as they've grown up and (b) the industry has changed. I'm not exactly sure which caused which, maybe the consumers drove the industry or vice versa, but the point is, games today are different than they were even in 2001. There aren't as many platformers, there are loads of first-person shooters, and people have started to care about weird things like "graphics". When you take all this into consideration and take another look at the Wii, it becomes clear: Nintendo is not trying to accommodate all these changes. Whether or not this is a good thing or a bad thing, I'll get to in a moment, but it's safe to say that Nintendo is not moving along with the general flow of the gaming world.
Some might say that they are stagnating and so the Wii U will be a flop, but I'm not so sure. Another way to look at it is that they're not dropping out of the race, they've just decided to slow down and fill the niche that they've always filled. If you think about it, their earlier systems really appealed to kids, or as we might call them nowadays, non-hardcore gamers. Many of these kids have grown up to be more "hardcore" gamers, and Microsoft and Sony have been attempting to keep up with them, but Nintendo hasn't. They've kind of let the kids outgrow their target market and now they're aiming at the next batch of kids and other non-hardcore gamers.
And so, a lot of us look at the Wii and are disappointed, because when comparing it to the PS3 or the Xbox 360, it really falls flat on its face. It's not even in the same league. But it's not meant to be. Nintendo hasn't lost the race, they've just swerved off to a different path that Sony and Microsoft barely notice.
As for the Wii U, it's just the next step of Nintendo down this path of a more "casual" gamer. And even though it bugs a lot of people, there are a lot of casual gamers out there. (Just ask Angry Birds or Farmville.) So while the Wii U probably won't be that impressive, I think it's probably going to be a success, assuming there isn't any major hardware and/or software issue at launch.
But the Wii U probably won't be just for casuals. People will buy the Wii U for certain titles (Zelda, most likely), or even just because it's cheap, just like the Wii's launch. Even though those people are probably also going to own some other system that they play more frequently, they'll help contribute to the sales for the Wii U. And that's what I mean by "success": will people buy it? Will it be a big enough market for game makers to make games for it? The Wii may not have been very impressive, but it was definitely a success, in that it was successful. My guess is that the Wii U will be equally unimpressive, but equally as successful.
(I can't believe I just typed a post about this. Generally, I hate people that try to speculate on things that there is no point speculating on. But oh well.)