Long-Term Exposure to Flat Design: How the Trend Slowly Decreases User Efficiency:

Clickable UI elements with absent or weak visual signifiers condition users over time to click and hover uncertainly across pages—reducing efficiency and increasing reliance on contextual cues and immediate click feedback. Young adult users may be better at perceiving subtle clickability clues, but they don’t enjoy click uncertainty any more than other age groups.

Common sense? Yes. Yet so many sites leave users guessing as to what they can click on.