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The "About This Mac" screen now displays "OS X" as the operating system's name, rather than "Mac OS X". The name change was made to differentiate "Mac" hardware from "OS X" software.
There's a new glass-like dock. It has a less reflective feel, and the glowing blobs below that indicate when an application is running now appears down the front side of the dock bar. The icons remain the same.
"Mountain Lion" now has notifications -- a further iOS-ification of the desktop and notebook operating system. If it's an important notification, it will stay there at the top of the screen -- though a snooze button allows the temporary subduing of the alert. Other alerts, such as tweets or Facebook statuses, will slide away after a few seconds.