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. This technique is called a "signifier": it signifies how to interact with the UI.
Visual designers, Creative Directors and UX designers have been on a mission for the past five years to "clean up" or minimalize UI design. The approach is called Flat Design. Design minimalism is not new and the intentions of de-cluttering your design are vital. However, industry studies and our own usability tests conducted on Web, Web App and Mobile apps tell us that there are a few things missing from most approaches to Flat design or new hybrids like "Flat 2.0".