As far as I know, the GNOME project has been the only Linux graphical shell that actually performs systematic studies of the user experience, and based on this, changes the interface. Testing how users are able to figure out a system is crucial, especially watching people (without helping them) who have never worked with Linux before.
This is a very different axis to most Linux developers building UIs based on what they feel like, which is most of what we read about on slashdot.
This is a very different axis to experienced users who have an emotional reaction when things change, which is where most of the software building energy goes.
GNOME indeed has a very nice interface design guide (likely originally inspired by Apple, but they evolved themselves), that you can consider for your own non-GNOME projects.
I like GNOME's systematic approach, and great to see that Germany is putting money to these efforts.