Computer technology long ago reached the point that "look and feel" can be adjusted to suit almost any tastes. And this in turn means that fashion trends can sweep through the computer field just as they do in other fields. One year it might be flat, pastel-colored icons, another year it might be photorealistic 3D icons, and so on. It's all highly subjective and a matter of personal preference.
The only time it irritates me is when the industry moves as a herd, as it is doing now for smartphones (Apple included). Just because some other vendor has decided flat icons are the Next Big Thing doesn't mean that every vendor has to immediately start using them.
In any case, if flat icons it must be, at least the ones on iOS look better than the ones on any version of Windows. Windows 8 is just the latest in a long line of mistakes for Microsoft, ever since Bill left. That's why I worry about Apple now that Steve is gone.