I always wanted a demo disc that had small portions of gameplay from many different situations. Each would only be a few minutes long (some would include multiplayer, though), but they would show exactly what the Wii is capable of. Demos from every genre, and some that don't fit in one, utilising the controller to make the owner understand what they have with it. These could also be great to ease people into using the remote so that they are better acclimated to it. In fact, they should put this demo disc on display in their booths, which I hope they have everywhere..
EDIT: They could also include some of the tech demos in here, from the TGS ones to the obstacle course..