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.

In fact this is an option: you can construct HFS+ filesystems which are case-sensitive and some are (I think the filesystem used for time machine is case-sensitive). FWIW case-insensitive-case-preserving (which is the default) seems to be the naming option which is least vulnerable to awful braindeath: case-sensitive is clearly purer but is ExtremelyVulnerable, while non-case-preserving ends up like THIS or requires magic hacks.

More importantly, there was a fairly significant cohort of people -- I am one of them -- whose first serious exposure to Unix was BSD 4.x. Many of those people were then terribly scarred by Sun's defection to SysV (the early Solaris 2s were just seriously grim). If, 10-12 years ago, you wanted a desktop machine which ran a BSD-derived system, which did not require you to spend a lot of time grovelling around in the guts of broken device drivers and/or did not suffer from minor updates which caused the real-time-clock to stop or something, which had a window system which was not a crappy Windows knockoff and for which a reasonable competent set of desktopy applications was available if you wanted them, then MacOS was the only option. Indeed, it was the only option even if you relax the BSD requirement. Linux clearly is a lot better now from that perspective (although based on my experiences with Ubuntu they still do not really understand why 'let's just completely change how everything works every two years' is not a great idea for users: the CADET software-development model is alive and well).