CDE had some great concepts. It was super consistent, dtksh allowed programming the GUI, fantastic built-in help. If it were not for the probably security-holy code-base, the missing creature comforts like anti-aliasing and dated look will keep me away from it.
Motif is also a bit chunky, and the serif font looks a bit dated, but it is very consistent, and has well designed colour schemes so very easy on the eye.
The dock was a really useful tool though, drag and drop printing and workspace short cuts. The version I used on HPUX even had a small green blinking activity light like a disk activity light (probably also network) which I loved.
