OpenGL Interface, Quick View, Dashboard
OS X’s GUI is running on an OpenGL backbone, hence, animations are extremely fast, and things move in a very fluid manner. Everything is rendered through OpenGL. Because of this, GUI duties are significantly less taxing on the main processor, and when you’re on the desktop doing work, processing for the interface is offloaded to the otherwise idling GPU.
One popular game such as Blizzard’s World of Warcraft, available for both platforms, actually integrates Fraps into the game, allowing players to record in-game videos as they play, without the need to install a utility. The point here isn’t that you can do exactly the same thing by using a utility in Windows, it’s that OS X is OpenGL, allowing much better resource control and other graphical features like built in Fraps.
One exceptional feature that’s integrated into Leopard is the ability to "quick view" many different file formats. I don’t need to open a PDF viewer to take a look at a PDF document. Just select the file and hit the space bar. A window pops up with the document fully displayed and scrollable. Quick View is compatible with many audio, video, and document formats and you can even Quick View multiple files of different formats at the same time.
Apple introduced the concept of widgets with the release of Tiger, and there are tons of useful as well as entertaining widgets that are free for use. Widgets are easily accessible from the Dashboard, which flows right onto your screen when you need it. Simple tools such as calculator, system status, and other things can be accessed from Dashboard. Google for example has a Gmail widget, which when launched, shows you your Gmail inbox without having to open a web browser or email client. One of the best system monitoring tools out there is iStat Pro, which details everything from core temperatures to fan speeds and other statistics.