The whole OS and almost every application looks and feels the same, as if a single team developed the whole thing, thanks to Apple HI Guidelines. Official guidelines for user interface design made it possible for users to actually use most Mac-applications in a very same way, creating a seamless and comfortable experience in the end. Users are able to anticipate how system behaves and what to expect from its applications. In fact, consistency dramatically improves learnability and usability of interacting with the system.
Installing and uninstalling applications is simply drag-and-drop. It can’t get much simpler and more intuitive than that. In fact, it’s hard to make any errors here, e.g. selecting some wrong option in a drop-down menu or clicking occasionally on the cancel-button. Quick and simple.
3. Effective and appropriate metaphors
Mac effectively uses the power of unambiguous metaphors. The different overviews in the OS just work. Exposé does the right thing, Time Machine uses a 3D view where appropriate (none of that 3D-flip ‘just for the sake of it’-nonsense of Vista). Depth in Time Machine represents the location in time and therefore uses a neat metaphor helping the user, and browsing your albums with Cover Flow in iTunes (and Finder) feels almost like the real thing." (Continued via Smashing Magazine, Juul Coolen) [Usability Resources]