Summary: Google's Material Design language reinforces one of the most valuable principles of graphic design and user experience: provide design elements that make it clear that a control is present for use.
The Problem with Clean UI's
UI's that are so clean, are invisble! Hiding UI elements is one of the worst things a designer can do. But the motivation is understandable: clean up the interface, reduce the clutter, simplify! These are all extremely important goals of UX Design and Visual Design, two overlapping efforts that go into design.
Image above: This playing card from our new UX Certification training (fresh dates to be announced in 2018), captures a key Material Design point: "Signifiers over clean". Clean is spelled with a K as in "Mr. Kleen" (a popular American kitchen cleaning spray) to remind us of the abrasive power of being clean.