I think there also is a conflict between interfaces that have a flat learning curve on one hand and interfaces that work extremely well once you are used to them. I think the second category gets too little attention. It's also much harder to user test the second category but for certain types of applications it's what should be preferred. Ideally you can have both a flat learning curve and high efficiency for experienced users. To me the JetBrains IDEs are an example of this compromise. Examples that are fabulous for experts are of course Vim and emacs
Having done research into UI/interaction design, I can confirm that all research in this area prioritizes things that (a) can be measured quickly and (b) with a high degree of confidence.
Good luck running a controlled experiment where multiple folks use an interface for an entire week and then become power-users of the system!
