First, I'm probably not alone on HN in reading that last sentence of yours that begins "Other than..." as being roughly equivalent to "Other than that, Mrs. Lincoln, how was the show?" OS X doesn't just have "some degree of POSIX-compliance"; it's Unix, full stop. If you are that "specific kind of user," this isn't optional.
Second, as squishy as this sounds, I simply like the OS X user experience more than Windows or any Linux/FreeBSD desktop environment that I've tried. The Mac gets the GUI right in subtle ways that are hard to describe but that I always notice when I'm using other systems. And this isn't due to lack of personal exposure; I'm, well, old by today's computing standards, and I've used the original MacOS for years, FreeBSD for several years (including professionally) and many versions of Linux, from the SLS days up through Ubuntu 12. (Actually, I'm running Ubuntu 14.10 and Arch Linux on two different servers, but they're GUI-free.) And I wouldn't trade OS X for any of them.
I know for some people, being able to tile terminal windows into a 3x3 grid without ever touching a mouse is their UX nirvana, but I am not one of those people, and I don't think I'm less productive for it. The fastest way to get a window the size and place I want it is often with a mouse. The fastest way to copy and move files is often with drag and drop. I know (some) people insist that I must be slowed down by constantly using mouse-driven software and switching between a tabbed terminal, a GUI text editor and a visual diff tool -- all with (gasp) overlapping windows! -- but I'm not. Really. And there are a fair number of Mac-only programs that I prefer to their Linux or Windows counterparts, if I can even find such counterparts. (Keynote, OmniOutliner, Soulver, ReadKit and xScope all come to mind.)
And, last but not least, it's nice -- at least for me -- to have a full Unix system that also has a lot of commercial software support. I don't run much Microsoft or Adobe software, but I'm glad I have the option. The applications I mentioned in parentheses there are all commercial, and as cliché as it may be to claim that commercial software generally has a better UX than free software -- and even more obnoxious, that Mac software tends to have a better UX than Windows -- it often matches my experience.
I fail to see the difference between OSX and Windows then. Legacy Win32 APIs probably are still there, somewhere, but they've built a fairly solid system on top of that. PowerShell is one major argument in favour of Windows platform: it really makes resource management and scripting (and remote administration) quite nice. Meanwhile, for practically anything other than core system services (and sometimes for those too...) you need to install 3rd party applications.
It's exactly the same on Mac OS: I had to install Spectacle just to make the system support the most obvious shortcuts for positioning and resizing windows. Homebrew/mac ports are nice and I know of no Windows equivalent, but I think they target very specific kind of users and are rather limited in usefulness.
In any case: I'm a Linux user, have been using FreeBSD on the desktop before that for years, worked on Windows earlier, and now I'm being forced to use a Mac for iOS development. I see no real difference between OSX and Windows7/8 in terms of OS capabilities: out of the box they're both rather weak (for my purposes, anyway). With some tinkering and 3rd party applications both can be made into workable systems - but the tinkering is both harder and more limited than what's possible on Linux.
Other than mentioned homebrew and some degree of POSIX-compliance which makes compiling many *nix programs natively under Mac OS possible, what makes it nowadays better than Windows (I mean core OS functionality)?