GUI should not be intrepreted as end-user; it should be intrepreted as a primary visual means the user or administrator uses to interface with the underlying system.

To that end, if the GUI isn't designed completely or well, then it doesn't matter if the person is a casual user, power user, or super user - if they can only get to 30% of the system's abilities, then its not done correctly - unless correct design follows some (general) assumption that seems to follow a lot of cultures

In my opinion: A well designed UI (Graphical, Virtual, Textual, or mix of all) does the process of enabling the user to complete simple actions efficiently, and endears those who want to learn more about the system a safe and accessible means of doing so (for example, Symbian - despite its dearth of menus, PalmOS - despite its programming and usage paridigm roots, or iPhone - despite its utter disregard for user-flow-paradigms outside of S. Job's.).