Mac OS uses the 'shrinking window' effect to show you where your window has gone, and drop-shadows to give you a sense of depth. Translucency in menus gives a subtle feeling of impermanence, and subtle transparency emulates the effect of paper-on-paper, where you can kind of, but not quite, make out the letters on the page below the one you're viewing.
Compiz, however, seems to be based on the philosophy of 'Now that windows are textures, we can do arbitrary transforms on them' and throws in a lot of effects that make little sense. Menus that wobble into place, windows that behave like they're made of Jell-O, and so on. They're neat conceptually, but I tired ages ago of people pointing out how Linux was better because I could configure my windows to burn up in a puff of smoke whenever I closed them.