I'm thinking strictly about the customization of the UI, not of "everything else" in System Preferences (e.g. hardware, network and so on). You can set individual icons but you don't have icon themes per se, can't change the UI theme (and I distinctly remember there was an application that did that very well back on 10.4, and 10.5 broke it along with most other efforts), can't change titlebar buttons and so on. E.g. I very much prefer the NeXT arrangement (close button on the right, minimize button on the left), because I really have no use for Maximize on a 27" screen. Good luck with that.

To their credit, though, Apple does at least think twice before pushing changes most of the time. Gnome's latest effort, with the client-side decorations, is maddening. Now every Gnome application has a thick titlebar that's also a toolbar, and every other application doesn't. They supposedly did that in order to save screen space - which could have just as well been done by reducing the thickness of that mammoth title bar Adwaita uses - but the effect seems to me to have been exactly opposite. Surely someone must have seen that the result is inconsistent on any system that runs a single application not included with Gnome -- say, for instance, Firefox! Or Chrome! Or Emacs! Or Thunderbird! Whatever.

I certainly understand that Gnome developers have a vision for their UI, but the uncommunicative way in which they impose it is pretty unpopular among users (like myself) who moved away from Windows or Mac OS a long, long time ago because they wanted more control over their operating system (well, that and no BSODs, which was a common occurrence at the time when I left Windows behind. Did you know Mac users didn't even have preemptive multitasking then? God I feel like a dinosaur now. Get off my lawn.)