The other way of looking at usability is the two dimensions:
Efficiency is "I have to grade 100 student papers each week for this entire semester. I don't care if I need a bit of help at the start of term to understand the UI, but I want grading each paper to require absolutely the minimum time."
Discoverability is the "I've never used Moodle before. What do I do now?" thing.
I think it is clear that both are important in Moodle at different times. (They are not mutually exclusive.)
Actually, we could break discoverability down into "known unknows" and "unknows unknows". In other words:
"I know that in Moodle you can have an activity avialable to some, but not all, studens. Now, how do you do that?"
and then the person who did not even know that was possible.
I suppose social networks (Twitter, Facebook, Google+, ...) are excellent examples of web applications that get people to use features they did not even know that they wanted.