The Nintendo Wii has an amazingly unique controller that allows users to play games differently, but the Wii's focus has been diverted by competition, which hiders its ability to be a niche system.
A gaming experience depends on many factors among which one of them is control. Arguably the Wii controller can be much more difficult to use than a standard controller, depending on the type of game. Sure, Nintendo has made many Wii-specific games, but many of them do not emphasize the gaming quality and instead rely on the revolutionary controller to sell the product. If the game has no depth, then it is debatable as to whether or not it is gaming at all.
One of the latest titles for the Wii is the Wii Fit. This involves an entirely new "control" method, and while it is pretty addictive at first, some would argue that the "gaming quality" is substandard and the replay value is a bit tedious. In fact, it is exercise, and generally that is not easy to repeat. All the praise to Nintendo for being revolutionary and trying to create a fun way to get fit, but too many novel ideas take away from any niche creation.
Many of the top-selling Wii games are also available on other systems, which have better graphics capabilities and more appropriate controllers for hardcore gamers. Other titles like Rock Band or Guitar Hero are virtually identical on all of the other major consoles, which really negates any "niche" that the Wii would have.
What the Wii has is niche potential. It has unique technology; however, it takes more than uniqueness to fill a niche. It has to be focused and marketed well, as well as provide a decent product that is geared toward a specific audience. Right now Nintendo has to compete with the other gaming systems by incorporating many of the popular game titles and appealing to a wide audience. If your goal is to please everyone, then you are not filling a niche.
The control method alone is not enough to make the Nintendo Wii a niche gaming system. In order for the Wii to hold a niche, it would have to create games that could only be played using their controller. Right now many of the games could still be played with a "standard" controller. The Wii Fit is actually a step towards the niche direction, but that is not even the emphasis of the system.
The Nintendo Wii opens a door for many new gaming possibilities instead of one unique, niche gaming style. It may provide a general niche experience, but it is not a niche gaming system.