I love it when a paintbrush moves across paper and leaves a swath of color that blends with the paint below. In that instance the tool is part of the satisfaction, the sense of accomplishment.

Software and hardware can have the exact same impact. They don't have to be transparent or disappear while you work; they can join you in your effort and give a sense of mastery, of skill and competence.

This is my sweet spot for UIs that I work on, figuring out how to make a tool that fits and enhances the workflow. And there's nothing else I'd rather do than watch and talk with people as they work and find the obstacles that interfere with this experience.