The biggest problem with the Drupal 7 usability process is communication. To quote Bojhan "I think its essential to recognize that with how the feedback process was handled during D7UX, we demotivated a crucial group: our core developers". Indeed. Lost in the several hundreds of comments in this or that d7ux.org blog post and getting no feedback which we were used to in the issue queue caused a lot of people to throw up hands and walk.
Again in Bojhan's blog post "in most discussions, developers assume that design proposals are ill-informed…" to which Eaton answers in the comments "and designers, in most discussions, assume that developers are visually illiterate and uneducated about UX matters".