Visual navigation of browser tabs
WebOS, PlayBook OS, and Android 4 all do a form of it, so it makes sense OS X Mountain Lion does, too. "It" is providing visual navigation of open browser tabs. There's a new icon button in the Safari tabs bar that opens a scrolling list of browser tab previews that you can use to move among them. There's also a new gesture you can use on your touchpad or touch-savvy mouse to activate the tabs view.