T he release of iOS 7 signalled the beginning of Apple’s era of flat design. As the name suggests, flat design is about removing unnecessary visual frippery; it makes everything look as if it’s laid flat on the same surface. That simplicity can be attractive, but if you can simplify things too much, then you end up with icons and other interface elements where it’s not clear what’s going on. A good example of that is the "hamburger" menu you see on a lot of websites, which appears as three horizontal lines on top of each other; there’s no obvious visual indication of what that icon does, and we only know what it is because we’ve seen it so many times before.
The other issue with flat design is that it’s not always obvious where you’re supposed to tap or click, especially if — like Apple did in iOS 7 — you deliberately adopt a very minimalist color palette too. The iOS 7 Control Center was a good example of that; with a translucent white background and very thin icons and controls in either dark gray or white, depending on their status it was hard to read or to see statuses at a glance.