Rather soon after it came out, I've been sort of an anti-Wii advocate. I truly believe that the future of gaming (in drawing in new players) lies in accessible interfaces and easily grasped gameplay. Easy to understand, but still needing skill and commitment to master. However, I don't believe that tight balance and responsiveness should be sacrificed for loose, but accessible controls.
For me, this is less about good game design, and more about selling a gimmick. Taking a look at motion control based arcade games like MoCap boxing and Mazan: Flash of the Blade, players are very excited when first playing the game and experiencing a brand new gameplay control, but the novelty wears off fast. Players are less likely to return to these games than games that rely on engaging gameplay than gimmicky control. This doesn't mean games with non-traditional (Joystick and buttons/etc.) input, rather, controls that integrate meaningfully with the gamplay. For instance, Dance Dance Revolution or Time Crisis.
Similarly, I noticed this with gamers who bought the Wii. There's only so much Wii Sports one can take before the novelty wears off. However, that's not the worst case. While unresponsive and "approximated" controls still retain a measure of fun in games like Wii Sports, I find that with games that require more precise control, like Mario Galaxy, "innovation" turns into frustration. This kind of design flaw becomes even more evident in games like Smash Brothers, which I had the unfortunate pleasure of experiencing on the Wiimote only.
However, that's not to say the Wii is without it's place, as these entry level games do offer a great balance between accessibility and gameplay. However, I think as these gamers slowly evolve to higher levels of gameplay, the same balance of controls and game design are not going to keep them as interested or involved as before. I believe this is one of the reasons that the Wii has the lowest attach rate at the moment of any of the seventh-generation consoles. This point was especially driven home to me when I looked through a list of all the Wii games available to the market and was astounded that the majority of them were US developed and licenses of IP targeted towards younger audiences. And while I have only touched a handful of these games, I can only imagine that most of them run off some sort of control gimmick. Gamers for the most part don't buy Wiis for the most part, parents buy Wiis based on reputation. However, that is just what Wii need to spread gaming across a broader audience. The problem is what happens when these players move past the initial gimmick.
However, the Wii Fit brings something new to the table. The Wii specializes in approximation and emulation of actions, and what this means for Wii Fit is the emulation of exercise. Or in the minds of users, the emulation of a very special space, the gym. While I personally think the Wii Fit is rather useless in terms of actual fitness improvement compared to actually going to the gym (or exercise you could do at home, without the "sugar coating" of Wii Fit. ie Push-ups), it's the mental emulation that is really appealing to players.
This is even more so for players living in dense urban landscapes who may not have convenient access to the open space and facilities for the actual fitness activities emulated by Wii fit. Doubly so for cultures that place a focus on public privacy. Having a personal approximation of a gym right in your house, while having a decently good time is very appealing thing.
This being said, I think that's the main reason that Wii Fit is going crazy in Asia right now. This is especially evident in Taipei, doubly so since all Wiis in Taiwan are either Japanese or American since there is no Asia version of the console yet. However, Wii Fit imports are still selling out like crazy. Just listening in on conversations at various game stores, Wii Fit is the hot item this holiday season, and when I queried a few shoppers and shopkeepers about their intentions, I got basically the same answers for convenience and the "self deluding" fitness experience. There is definitely a place for the Wii Fit as a fitness advocate, but not as a game, especially not attached to a system like the Wii.
What I'm really getting at here though is the Wii Fit is exactly what the Wii doesn't need right now in order to shed it's reputation as a gimmick machine. While to the general public, the Wii can do no wrong right now, I think it's going to be going down the road of the Gamecube in the future, with only a handful of titles keeping the machine afloat unless more good developers sign up on board. And none of these titles will require the control scheme that makes the Wii unique.
One thing that worries me greatly is the announcement of the huge Japanese hit series Monster Hunter coming to the Wii for it's 3rd installment. I am very interested in how Capcom handles the design of the sensitive control scheme (as in if the Wii's motion controls are going to be relegated as a additional option instead of the main control choice) of the series is going to be the pivotal point for future Wii design. Good design informs good gameplay, just because the control scheme may be new, unique, and exciting doesn't mean that it will withstand the test of time, or sales.