Comment by pavon

Yeah, the icon style is the part I care about the least when taking about flat design. It is the fact that it removes all visual affordances that something is interactive. Clickable items that don't look like buttons or hypertext, they just look like the rest of the page. Scrollable regions that don't have scrollbars or any other visual clue that there is more content below or to the right, unless some of that content happens to be cut off by the invisible region boundary. Windows whose only indication of which is selected is the title goes from dark gray on light gray, to medium gray on light gray.