Three off the top of my head: the contrast of dark grey for focused apps to light grey for unfocused ones means that you can see what's focused instantly. This is useful because Apple encourages opening multiple things at once: the green button for maximize sizes most apps so they fit whatever's being displayed and nothing more.
There's no unused pixel. KDE 4.1 shown here has large blocks of matter at the edges of their windows. OS X does not. Likewise, that little grey button on the top right, the one that minimizes bars that you don't need, allows for much more efficient screen economy. No, you don't care, because you like looking down your lengthy xmonad nose at us common serfs, but a lot of us LIKE doing a lot of different things with our computer, and Apple doesn't compromise: we get an efficient system AND one that, you know, does stuff.
Not to reel off Apple's most recent ads, but Stacks mean you can access a lot of information without cluttering your screen, and you can still keep it all organized. Making Downloads into a stack means that suddenly your desktop is blank except for things you drag off from other apps and screenshots you take. It compartmentizes your computer into Downloads, which are things that take a little while to get, Desktop, which are things that you grab quickly, and Documents, where things are organized.
Three given as impulse. I could literally spend hours going into fine detail about the small little bits of polish that Apple has applied to their operating system. You can say a lot about OS X. You could argue that it's not "efficient," though by some definitions of efficiency it outstrips Linux and Windows by far. But you can not argue for its not being polished. Apple is the industry MASTER of user interface, they have perfected the art of polish, and if you say otherwise you are a stupid, ignorant fuck.
I say that with all due respect; I'm sure I'm overthinking your little flippant remark. I upvoted you because your comment leads to a venus of discussion - but you're about as wrong as a nerd arguing operating system on a Y Combinator site could possibly get.