To me, Tim, people to whom this applies:
are excellent examples of web applications that get people to use features they did not even know that they wanted.
Can be broken into several sub-groups, people who want to expand their knowledge and are deliberately curious, people who are willing to try things because of a lack of fear, people who do not want to try things until someone tells them that this might be useful, people who click the wrong buttons, who really have no intention of doing anything but are clumsily stupid enough to get it wrong for a, what is to them, positive result, and people who are seriously intellectually lazy and subject to mass marketing policies that at some subconcious level promises to protect them from the evils of the world if they click this button. My experience, somewhat cynically, is telling this last group is the largest - (Douglas Addams or Terry Pratchett would have put it a lot better of course.) When you seriously examine what the definitions above are expressing, they are basically returning us to the very point I tried to make earlier, there is a strong connection between "intuitive" and "familiar".
However, this does not overcome the fact that the sheer complexity of Moodle is not attractive to many potential users, they use the UI as an excuse not to use the product and are quite willing to accept a lesser product because it "feels" better. You and I both know that Moodle can appeal at so many different levels, from a simple, basic usefulness to a serious power tool. All I am suggesting is there has to be a way of developing Moodle where a user can be introduced to Moodle at the basic level and entice them into using more and more of its tools and strengths.
I am just not sure Moodle can make these kinds of changes to develop this kind of visual literacy now before another tool comes out of nowhere and becomes the next killer app.