What is the name of the set of features that improves the visual appearance of windows?

This set of features is called Windows Aero. It is available in Vista and Windows 7 only. The key characteristics of these features are the following:

*Window borders are translucent, such that you can see the window behind it through its background
*When you hover the cursor over a tab, it shows its contents
*When you press Alt + Tab, it shows thumbnails of your active programs
*WIndows animation when opening or closing 

The new feature in WIndows 7 is the Show Desktop icon located in the bottom right of the screen. When you click it, all open windows disappear to reveal the desktop page.

Because people are wasting it