Principles of UI, A Thread:
1. natural mapping
2. visibility of system state
3. discoverability
4. constraints and affordances
5. habits and spatial memory
6. locus of attention
7. no modes
8. fast feedback
9. do not cause harm to a user's data or through inaction allow user data to come to harm
10. prefer undo to confirmation boxes. For actions that can't be undone, force a "cooling off" period of at least 30 seconds.
11. measure using Fitt's, Hick's, GOMS, etc. but always test with real users.