"A good tool is an invisible tool. By invisible, I mean that the tool does not intrude on your consciousness; you focus on the task, not the tool. Eyeglasses are a good tool -- you look at the world, not the eyeglasses. The blind man tapping the cane feels the street, not the cane. Of course, tools are not invisible in themselves, but as part of a context of use. With enough practice we can make many apparently difficult things disappear: my fingers know vi editing commands that my conscious mind has long forgotten. But good tools enhance invisibility."
Hill points out that one of the times we actually do notice technology is when it breaks. He also has a rather clever blog, Revealing Errors , in which he and other contributors "reveal errors that reveal technologies" so as to learn how they affect our lives.