Well Silks you really have to look at it in a different angle usually in music games you usually try to beat a predetermined score that would count someone "passing" the song and then mastering the song with a 5/5 stars, AAA(A), S rankings and so forth, the problem with that is that everything is predetermined and your trying to nail all the notes that developer wants you to nail.
Wii Music in a way is similar but executed in a way different manner, since the clear reward/end goal is really transparent. The goal of Wii Music is to make the song "sound good" or pleasant. But you don't have that much guidance or structure instead Wii Music invites people to find that structure and once you do find it, the player will be rewarded with a song that sounds right or awesome.