I kind of disagree with Threi about Wii having no clear demographic for which to cater. Surely, there are many different consumers who are buying Wiis, but why they are doing so is limited to the consumer values Wii, as a platform for games, offers. And reaching the broadest possible audience, I think, has more to do with seizing these values than developing with any one demographic in mind, which is the mistake, I think, of many third-parties on Nintendo's platforms.
In other words, developers will be more successful, I think, if they focus more on why rather than on for whom. Instead of having any one demographic guiding their approach, developers should create games which seize on the consumer values of the platform. This is what Nintendo does. NSMB wasn't developed, I don't think, with any one demographic in mind, but rather seized on why consumers bought a DS in the first place: immediate, tactile (that is to say, accessible and responsive) fun.