I'm in roughly the same camp as you here. I depend quite a bit on things generally working in a Unix-y sort of way. Not necessarily everything, but POSIX compliance is pretty important for me, since it means I can rely on a wealth of software that's already been made, in particular for command line use. Time Machine, disk encryption, and the applications already available for OS X are excellent as well.
Font rendering is also an important point of OS X for me. Generally speaking -- qua someone judging the OS based on whether I like looking at it every day -- Mac OS has the best font rendering. This in turn makes it much easier to stare at code, a terminal emulator, and many other tons of text all day. This is even better with the HDPI display on the recent Macbook Pros, which makes the screen really comfortable to use.
One of the biggest point for me is that Mac OS comes with bash and a terminal emulator. The flexibility that a shell on BSD, Linux, etc. offers me is far beyond what I could do with the Windows console back when I still used Windows as my primary OS. This is also somewhat more important on Mac OS, as Finder, the file manager (for those who don't know it), is absolute garbage. Windows still has the best graphical file manager I've seen, but I preferred to get around in the shell, so the Windows console was always inadequate.
That's probably changed a lot since then, especially with the advent of Powershell, but I'm still not convinced that Windows is any good for someone who relies heavily on a shell. Piping output into other programs and treating them as little functions to build up bigger things has saved my ass a lot. It's saved me often enough that I don't want to end up on a system that has a passable shell but lacks all the tools at my disposal on Mac OS or Linux. As such, I'm pretty committed to any OS that offers that same level of flexibility, and I've never gotten the impression that Windows could give me that.
Additionally, I also love Apple's APIs, at least Cocoa. The older C APIs are far less friendly, but they work and offer some very powerful tools as well. So, working in Objective-C (or MacRuby) is the way to go for me for a lot of personal projects. This is largely the only thing that keeps me on Mac OS now aside from it simply having all the software I know well.
All that said, it's troubling how much harder it's become to use all of my tools as I'd like. For example, in Mac OS 10.8, the ~/.MacOSX/environment.plist file, used to set environment variables in applications launched via Finder and such, was deprecated. I can work around this by editing a system file, but it's not a good fix, and I imagine it's only going to get worse. So far as I know, that sort of thing wouldn't happen on Linux, and is largely a result of Apple locking down large chunks of the OS, I think.
So, yeah, Mac OS for me, but only for as long as Objective-C remains mostly unusable on other platforms. That may be a long time, though, so I'm probably in this for the long haul. Either way, for the majority of the work I do -- mainly writing articles and software for my own use -- Mac OS is my preference.