Designers of natural user interfaces focus on how humans interact with their environments through their senses (touch, vision, speech) and consider how sensory interactions impact cognitive processing and creativity. For example, despite the development of many excellent paint programs, there’s something intrinsically more satisfying about using your hands and paint brushes/pencils to sketch something. There’s some sense of immediacy when you push an object on a screen and it moves out of your way.
Some design advice for creating interfaces is offered by Ergosign in this Slideshare presentation.