When Human Computer Interaction is not about the GUI

Many of the biggest advancements achieved by web applications are not achieved by the graphical user interface (typically web-apps are more limited than native applications in this area), but rather the shift from a single-user interaction (like Word, Excel, etc) to multi-user interaction (ebay, paypal, friendster, flickr, etc) -- that is, the human-computer interaction is primarily human-human interaction, mediated by the computer.

Ebay, for example, still has a very limited graphical user interface -- essentially text with pictures -- but is arguably the most successful web application ever. Similarly, Craigslist is nearly a terminal-based application (text-only), but is immensely useful and popular because of its ability to mediate interaction between people.

Since these applications are special for their multi-user interactions not their graphical user interfaces, a good web-application prototyping tool would focus on demonstrating multi-user interactions more than building a good GUI.

Regarding specifics, I think this problem may be similar to the one schema building/analyzing tools face, where, the problem is primarily defining relationships between data elements. In this way, starting with a SQL schema, an additional set of rendering hints could be maintained from which a multi-user web-app prototype could be generated and tested very quickly.

This idea was an offshoot of my Flash improvement ideas.