3. Are human beings aware of the capabilities of different media?
Human beings in general, and computer interface designers in particular, are not really aware of the capabilities and limitations of different media. Early computer output was restricted to text. This is a medium that most people understand, since it has been used for hundreds of years. Therefore we have an intuitive understanding of how to communicate with text, at least at a fairly basic level. When graphics became possible on computers, the situation did not materially change because graphics had also been used by human beings to communicate (often in conjunction with text) for many years. When colour became available, however, the situation deteriorated. Colour was used in a totally indiscriminate manner in early interface design. It was not surprising that the quality of interfaces deteriorated. Most people cannot choose their own wallpaper or clothes with a real colour sense. It was much later that designers realized that the spare use of colour was the key to good interface design.
It is quite interesting to reflect on why human beings (in the Western world in particular) do not have good colour sense. Mary White  makes the interesting point that "imagery" (which in her definition encompasses paintings, sculpture, and television as well as computer graphics and video) has been used as a primary learning tool for centuries. The invention of the printing press changed all that, and the primary mechanism for learning became the printed word. Ironically, towards the end of the twentieth century we have moved back to a world where imagery is important again, and it has replaced the word as a major political communication medium. There is, however, little research on imagery and its effects.
We are now about to enter the era of multimedia interface design, and I expect the experience with colour to be repeated, writ large. The quality of home videos testifies to the level of most peoples' fitness for designing inter faces using moving pictures and voice! Since human beings are only vaguely aware of the differences between media, we need a methodology that helps to answer the question, "What medium when?" to achieve a particular interface goal. I will return to the issue of the methodology later.