If you're one of those people who wants to play Rock Band but can't quite get the rhythm, even on Easy, then you are going to love Wii Music.
Unveiled during Nintendo's press conference, Wii Music is the music game for the coordination-challenged. While most other music games require a certain amount of precision and timing in order to succeed, Wii Music strips all of that away. To play one of the more than 50 instruments in the game, you simply mimic the appropriate real-world motion.
Shigeru Miyamoto himself demonstrated how to play the saxophone: just bring the Wii Remote up to your mouth and hit the keys in the timing of your choosing. For violin, extend the Nunchuk out with one hand to replicate the neck of the violin, then saw back and forth with the Remote for the bow action.
The Wii matches your movement to notes that would be appropriate for the song you're trying to play, making it literally impossible for you to fail. That doesn't necessarily mean you're guaranteed to make beautiful music, however. Some of the notes Miyamoto played during his demonstration definitely sounded a bit off, but given that he was, for all intents and purposes, air saxing, the music he produced was remarkably good.
Wii Music also has a separate, more intensive drumming mode, that uses the Balance Board as a foot pedal to go with the drumsticks of the Remote and Nunchuk. You can just drum away, or use it as a tutorial and actually learn how to drum. Definitely point this mode out to the children of people you dislike.
Making music is certainly a huge part of the appeal of Rock Band and Guitar Hero, but so is the score, and the feeling of accomplishment that comes from advancing through more and more difficult levels. Wii Music doesn't present those kind of challenges, but it removes the feeling of frustration that less skilled players get when they fail over and over. Is that enough to make it a runaway hit? Quite probably, yes.