Don't be so dramatic. From a usability perspective, it works enough. Also, interactivity is changing and themes are options. People don't hover or click with touch devices.

Many UI concepts are in flux (again). Link? Button? Collapsible? Let go of what you know and watch people using flat UI.

There are many ways to hint interactions within flat design. As has always been, stick to more abstract interaction fundamentals and processes and don't wed yourself to fads based on current (or old) tooling trends.

It's easy enough to use color, contrast, font, positioning, size, shape, etc. all to indicate interaction points. That's all a gradient button is compared to the text in an old message box.

Also, there are two major thrusts in flat ui that make it compelling. One is the additional focus on content since the UI takes a step back once you spend less time tweaking artificial details. Which leads into the second point others are making. The time and energy spent in details to achieve fake results in a digital context are dishonest at best and grossly out of place from a design perspective. There's a little bit for everyone in there.