This animation shows the upper-left corner of the Mac interface. The Apple Menu is gone, replaced by an application menu. The Desktop’s application menu lets you set some system-wide preferences, as well as log out. In every Mac OS X-native application, the Quit command has moved to this menu.
In addition to the smooth, anti-aliased text on display here, this animation shows you the transparency of Mac OS X menus (you can see through just below About This Mac to the red-yellow-green window buttons beneath), as well as the very cool “fade-out” effect when a menu goes away.