One significant area of non compliance with unix conventions is its non case sensitive filesystem (HFS and variants like HFS+ if I recall). I think this is partly for historical reasons to make Classic / MacOS9 emulation easier during the transition. But I could never understand why they did this, they could have put case insensitivity in their shell and apps without breaking the filesystem.
Anyway despite its being unix I can't really see it gaining much traction with serious unix users (when did you last get a 404 from a major website with a tagline "Apache running on MacOSX"?), the MacPorts and Fink repos are a really bad and patchy implementation of something like apt/ctan/cpan/etc (I think possibly at least one of those repos builds from source with attendant advantages/problems), it does not support X properly, the dylibs are non standard, everything is a bit broken compared with Linux (or FreeBSD) and Apple does not really have the motivation or the manpower to create a modern, clean system like unix users expect.
Open sourcing Darwin was supposed to open it up to user contributed enhancements but Apple was never serious about this, it was just a sop to people who claimed (correctly) that Apple was riding on the back of open source and giving nothing back to the community. Since Apple refused to release any important code like drivers or bootstrap routines the Darwin release was never really any more useable than something like 4.4BSDLite. People who loved their Macs and loved unix and dreamed of someday running the Mac UI on top of a proper unix, put significant effort into supplying the missing pieces but were rebuffed by Apple at every turn, Apple would constantly make new releases with even more missing pieces and breakage and eventually stopped making any open source releases at all, leaving a lot of people crushed and very bitter.
As for me I got on the Apple bandwagon briefly in 2005 or so, at that time I was experimenting with RedHat but my primary development machines were Windows 98 and 2000 (occasionally XP). My assessment was RedHat was not ready for desktop use, since I had trouble with stuff like printers and scanners that required me to stay with Windows (actually this was probably surmountable but I did not have the knowledge or really the desire to spend time debugging it). That's why I selected Apple as a "compromise unix" which should connect to my devices easily. I got enthusiastic and spent a good $4k on new hardware. Shortly afterwards Apple announced the Intel transition so I realized my brand new gear would soon be obsolete and unsupported. I was still pretty happy though. Two things took the shine off eventually (a) I spilt champagne on my machine, tore it down to discover my beautiful and elegant and spare (on the outside) machine was a horrible hodgepodge of strange piggyback PCBs and third party gear (on the inside), this apparently happened because options like the backlit keyboard had become standard equipment at some point but Apple had never redesigned them into the motherboard, the whole thing was horribly complicated and fragile and never worked well after the teardown (b) I got seriously into FreeBSD and Linux and soon discovered the shortcomings of the Mac as a serious development machine, everything was just slightly incompatible leading to time waste.
Happily matters have improved a lot. Lately I was setting up some Windows 7 and 10 machines for my wife to use MS Office on for her uni work. Both had serious driver issues like "The graphics card has crashed and recovered". And on the Windows 10 machine, despite it being BRAND NEW out of the box and manufacturer preloaded, the wifi also did not work, constantly crashed requiring a reboot. Windows Update did not fix these problems. Downloading and trying various updated drivers from the manufacturer's website seems to have for now, except on the Windows 7 machine where the issue is noted and listed as "won't fix" because the graphics card is out of date, the fixed driver won't load on this machine. Given this seems to be the landscape even for people who are happy to spend the $$ on the official manufacturer supported Windows based solutions, Linux looks pretty easy to install and use by comparison. Not problem free, but may have fewer problems and easier to fix problems.
It appears to me that with the growing complexity of the hardware due to the millions of compatibility layers and ad hoc protocols built into it, the job of the manufacturers and official OS or driver writers gets harder and harder, whereas the crowdsourced principle of open source shows its value since the gear is better tested in a wider variety of realistic situations.
cheers, Nick