I love Linux, and have used it in some form since shortly after it came into existence (and used various flavors of UNIX before that). These days I mainly use Linux on servers, mostly command-line only, and use OS X as my primary desktop OS. I've used Linux on the desktop plenty in the past, but don't do much of that anymore (very occasionally use Ubuntu on my ARM ChromeBook, that's about it).
It actually took me a while to prefer OS X, and initially I preferred Linux. Some of that was not knowing OS X well enough, some was not finding the right 3rd party tools to make things more comfortable (e.g. ShiftIt, Butler), and some of that was the OS X developer ecosystem improved dramatically over the years that followed the switch to Intel (e.g. Homebrew coming into existence to replace MacPorts and Fink). Also, OS X 10.5 added great features, and the updates since have been gravy.
Why do I prefer OS X? Well, to begin with, I can do all the UNIX-type tasks that I can do in Linux. But that's not enough to pull me away from Linux. As a hobby, I record music, do music production, and do the occasional video editing. For music, I used GarageBand, then Logic, and now Ableton Live (and Native Instruments Komplete). For video editing, I used iMovie, Final Cut Pro 6/7, and now Final Cut Pro X. Though there are options in both of these areas on Linux, I don't think they are as good, though that could change in the coming years (maybe Bitwig Studio will finally get released and plugin developers will give some attention to Linux; and maybe Lightworks for Linux, currently beta, is a good option for video editing). I also use Aperture for photos, though I could live something else.
For work (I'm a developer), OS X also benefits from it's larger desktop market share, and thus gets commercial support in ways that Linux doesn't. Examples: A while back I did Flex development, and Adobe dropped support for the Linux version of Flex Builder. GotoMeeting doesn't have a Linux version of the full client, and I doubt you can get to work with WINE. I occasionally use the OS X version of Microsoft Office. I also occasionally play around with iOS development. That said, for most development tasks, I would be fine with Linux, and I run a Windows VM for a few things anyway, so I could use Windows versions of things not available for Linux, though it wouldn't be as convenient as Mac native versions.
There is also the "just works" factor, especially when it comes to laptops and power management, and Mac hardware is fantastic. I'm currently using a 15" Retina MacBook Pro at work, and it is a fantastic developer machine: light, portable, powerful, and the way OS X handles the Retina display is great (scaling options, etc). Also, I run 4 displays off this machine when I'm at my desk (laptop display + 3 externals), and connecting/disconnecting external monitors is flawless. Thunderbolt saves me from having to connect some of my cables every time.
I know you said leave Windows out of it, but it is a factor in my case: for my home/personal use, I would be running both a Windows and Linux machine for what I need (virtual machines wouldn't really do the job for either OS) -- I actually used to do that with a KVM to switch between two machines; instead I can just use OS X for all my personal use needs; I don't have Windows installed at home at all. For work, I require Windows, and for my work-related tasks (which includes Visual Studio), Windows runs perfectly well in a virtual machine (and is actually easier to manage that way). BTW, I haven't tried it myself, but I've always heard OS X doesn't run well in a VM (so Linux as host, OS X in a VM is not practical).
Ideally, I wish the open source option was the better fit for me, and someday I may return to Linux, but for now OS X is the better option for me. YMMV.
