Both of my three year old grandsons 'wag' a leg when they get excited and want to tell us something funny. In adults I've always looked at it as a sign of impatience or simmering anger, but maybe it isn't always.
My own legs have for years been 'dancy', especially in airplanes. An internal tickle makes one leg or the other jump up and down until the tickle goes away. The tickle is a purely physical reason for my jumpy legs, and probably connected to PD, but in general, I think legs must express more conscious and unconscious emotion than we are aware of.

birte