I maybe have a more "classical" (for lack of a better term right now) understanding of lightness and harmony.
I suppose to me harmony and lightness is something that I strive for in my "Scout and Me" time. I cue him as gently as possible and as firmly as necessary, and he responds with the level of lightness that he's willing to give to me that day. Harmony results from the "sharing" of the "maxed" lightness.
Lightness and harmony are works in progress, as well, for horses and riders. Novice riders and trainers generally haven't built up the experience and feel to maximize their lightness, so the horse can't maximize his yet. Green horses don't understand yet how to give, and can't reciprocate an experienced trainer's lightness until that lesson is learned. Lightness and harmony are things to always strive for and work toward, but I think that relatively few horse/rider teams have achieved it in the grand scheme of things. I know I haven't yet.
I remember one lesson, back in my WP days, I rode a 13.2 hand paint pony mare, and we were both "on" that day. I had her loping (true 3 beat, but beautifully slow and smooth), draped rein, her face soft and on the vertical, poll just over wither height. We did sort of a reining-style "lope U-shape, stop, back, rollback, lope depart, repeat at other "branch" of the U" pattern. That was a really harmonious ride that really made an impression on me on how light and soft horse and rider both should be. I hope that Scout and I can replicate that same kind of ride someday.
Good topic, Spirithorse! I look forward to reading more responses!