Take Vista and Windows 7 for example, you see the aero Window edge is roundish (especially top two corners). Now take look at Windows 8, as it is pretty edge, not round at all. Do you know why? It is really because Windows team used at least 5 rectangles, all stacked together to form that roundness for Vista & 7. That means it is more GPU cycle time to draw these duplicated rectangles to give such pretty illusion. With Windows 8, I do welcome the edge/sharp corners, feel more clean and snappy.
Then again, I'm a developer, not a designer...maybe these trends (such as the apple-esque image reflection with a fade) will actually stick around long enough to be useful in the CSS3 spec.