The 8 Golden Rules

Consistency: naming, color, font, ...
Shortcuts: speed for experts
Feedback: the frequent the action the more detailed
Closed dialogs: Start-middle-end -> structured work (finish something)
Robust against errors: Users must not be able to make a serious mistake
Undo: Correct mistakes, be able to try out everything without fear
Control to the user: User understands what the GUI does to the system underneath. The GUI does not restrict the user.
Reduce short term memory usage: provide on-line help

Help, Documentation and Feedback from the GUI

Help and documentation is part of the GUI and has to be adequate for the users:
Provide Tutorials for beginners and well structured reference books for experts (on-line).
Customize the GUI feedback to the users.