What I'm talking about would be a game where the progression of motion, postures, and gestures would look more like a smooth ballet or practicing of connected katas. The entire game would be an act of learning how to connect the motions and learning which motions, postures, and gestures naturally transition into each other such to create a singular, practiced flow of motion. Precision of angles, speed, and timing would all be important to the movements. Such a game could not only be entertaining but also provide great exercise, as well as improve dexterity and coordination. One could use such games to do things like actually learning how to dance, learning some martial arts, or, as mentioned, just engaging in some fun exercise(imagine a sci-fi space shooter with some rhythmically pumping trance or electronica that you have to actually dance to control, and I mean really dance, not the elephant-stomping DDR-style dancing).
So, in summary, motion controls generally suck because they are being used as simple mappings to traditional button-pushes rather than being used as the intrinsic new paradigm of control that they actually represent. One has to think outside the very small box of current traditional mainstream game design.
