During the E308 conference, it was announced that the game would be available without the Wii MotionPlus add-on, but will still translate motions similar to those used to play real instruments. Therefore, how you move the Wiimote and Nunchuk, along with the timing in which you press the buttons, will affect how you sound. You can also use the Balance Board for certain instruments, like the drums (where it's used as a pedal). That's probably good news for those of you who were worried about more Rock Band-like peripherals clogging up your homes.
Wii Music was made with people who can't play instruments in mind, so that they can "experience the joy of making music," too. Since Miyamoto wanted this game to be accessible to the music illiterate, it doesn't require any note reading and won't rate your performances.
Also interesting is that songs can be recorded and made into music videos (starring your Miis). Furthermore, the game supports up to four players (per Miyamoto), but we were slightly confused when it looked like five were performing the Super Mario Bros. theme on stage.
Being the competitive music junkies that we are, we're a little bummed that the software doesn't even give you the option of performance evaluation. Even so, this looks like a pretty rad piece of kit.
[Update 1: Screens added! See the gallery below.]