I am studying open source usability in my master's thesis (with my work on Moodle as a case study). I am very curious about how do you go about getting the design right in WordPress? It is just so good. The literature I have found on the topic mostly seems to repeat the fact that open source usability is hard due to a number of reasons.
A key reason certainly seems to be keeping the design narrowed down to a specific purpose - and having a plugin architecture to allow people to fill in. But keeping the feature set small is an art of its own. Do you use any of the standard User-Centered Design stuff like Personas or Scenarios? What is the quantity of usability testing you do? Matt has mentioned that he employs usability experts from inside the community though, but there does not seem to be much actual discussion about getting to know the users in the documentation or the forums.