UX Principles
Affordance
Visual cues that indicate how an element can be used. Buttons should look clickable (raised, colored, labeled). Links should look tappable (underlined, colored). Flat design often destroys affordance — add it back with shadows, borders, or underlines.
In plain terms
Visual hints that show how to use something — a button should look pressable, a link should look clickable.
Why this matters
If controls don't look usable, people don't use them. Stripping away visual cues — flat, ambiguous design — leaves everyone guessing, and hits users with cognitive or low-vision needs hardest.