I think the concept of Mii's is fantastic! There was almost as much time spent this past week creating Mii's as there was playing Wii Sports.
I would like to see the Mii's intereact with each other more. I think that when a game 'save' file is created, a new 'Activitii' should be added as well. For example: With Wii sports a few mii's could play tennis. In Zelda, they could ride horses or sword fight with each other, and in Mario (when it comes out) they can jump on each-others heads.
This could be Nintendo's "Achiivment" system.