As I have mentioned before, the Drupal project takes a very conscientious approach to usability. Unlike many commercial and open source software projects, Drupal enlists the help of usability labs and is very receptive to negative feedback. That is not to say that Drupal has achieved usability nirvana. They struggle with it like everyone else - especially as they try to add functionality and flexibility into the system. It is just that they practice their usability work so out in the open.
There is an opportunity to see the Drupal usability process first-hand by participating in this call to action from usability consultants Mark Boulton and Leisa Reichelt. There are several ways to participate. For more details, visit http://www.d7ux.org