In '94, a magazine editor complimented Graphing Calculator 1.0, saying I should be proud of the elegance of its user interface, despite it being easy to program. The implication that we had succeeded in hiding the enormous complexity under the hood providing the user with the illusion of a simple tool was deep praise.
As I've preached elsewhere, making powerful software easy to use and easy to learn is enormously difficult. It takes people with an incredible combination of skills, talent, and artistry working together with intensity and patience. When done successfully, the ratio of (actual effort) / (apparent effort) can be astronomic, with its huge numerator and tiny denominator. I take a perhaps perverse pleasure in working on things which are so unobviously difficult.
Slacklining is satisfying in the same way. It is like learning to walk all over again. I have great sympathy now when visiting friends with toddlers taking their first steps. It took me a month to stand on the line without falling, then another month on my left foot. After a year, I can turn around (clockwise, but not counter-clockwise, yet.) I still cannot stand still for more than a moment. Yoga teachers say that tadasana, the basic standing posture, is the most difficult pose. Standing still on the line is an intense workout. It fully engages the core muscles, the quads, the calfs, the shoulders, hips, and arms, and all the small fine-motor control stabilizing muscles. It occupies ones full attention as well, while your eyes, inner ears, and brain try to maintain balance. The more work you put into it, the more it looks to an observer that you're not doing anything at all.