Usability failures include low contrast, no delineation of borders, and no visual cues for the interactions that are permissible for different parts of the interface (e.g. "clickable" button vs. bare text).
Sigh. I like the aesthetic but fad-monkeys overwhelmingly fail at the engineering part of design. You can be simple and elegant without going backwards on other usability principles.
Buttons didn't have gradients or 3D effects or often even colors for nearly 10-15+ years on Macs and PCs and people were able to use them just fine. The did often have highlights and borders, however. A button in Classic Mac OS was unmistakably a button without most of those cues.
It's possible to have a nice, useable flat design. However, great care must be taken to use visual cues and be consistent in the UI. It's also possible to have high contrast. Low contrast is not flat design, it's a poor implementations of colors with a flat aesthetic.
I noticed that facebook backtracked on their search bar. When it first launched it was completely non-obvious that it was a text input. A few weeks later they added a negative emboss effect. I agree, it's a nice style, but I think we'll see it settling on a middle ground regarding 3D effects on UI elements. I also think some of the more extreme proponents of the style may be younger designers that haven't already been through several of these cycles. :)
Sigh. I like the aesthetic but fad-monkeys overwhelmingly fail at the engineering part of design. You can be simple and elegant without going backwards on other usability principles.