The purpose.
WHAT IT ACHIEVES
People accomplish what they came for With less effort With fewer errors Without needing instruction
WHAT IT IS NOT MEASURED BY
How it looks in a portfolio How current the style is How much it impressed the person who commissioned it
WHAT TO MEASURE INSTEAD
Whether people complete the task How long it takes Where they fail What they ask support about
THAT LAST ONE
Support requests are design feedback.
Repeated questions about the same thing indicate an interface problem.
WHAT GOOD DESIGN LOOKS LIKE TO A USER
Unremarkable. Nothing gets in the way.
WHY THAT IS DIFFICULT
Invisible work is hard to justify and hard to praise.
WHAT BAD DESIGN LOOKS LIKE
Hesitation Errors Abandonment People asking how to do something obvious
WHAT TO OPTIMISE FOR
The task, not the impression.