The Darwin kernel is regularly updated; for example Apple replaced Unix init and cron with launchd in 2005, achieved full POSIX compliance in 2007, and went to full 64-bit in 2009.
I don't think this is true at all. Sure, there's always been a BSD-style environment for compatibility, but Apple has never been shy about ripping and replacing legacy Unix-y infrastructure (see: launchd) or about adding new, "deep" APIs close to the plumbing (see: libdispatch).
Meanwhile, you make woolly justifications of their slowness by making contradictions like this: "They're not slow to add features. Some may arrive after a competitor" Comments like these are not helpful and do not add anything to the conversation.
They're slow. The reasons why this is the case is still not entirely clear. But even OSX fans get frustrated sometimes at how slow they are to add features that other operating systems got years before. The most vocal recent one I can think of was probably the TRIM support for SSDs.
I always have a normal user account and an seperate admin account. The normal account does not have sudo permission. This setup on Ubuntu and what I can remember on OSX is not hassle free because a lot of installers and apps assumes normal user has some sort of sudo.
Unfortunately Windows 8.1 dumped UAC in the toilet when forcing the Windows 8.1 update to be downloaded from the store, which made your local admin account useless.
On OSX you get an elevated privileges login prompt for any installer needing to write to / and even for many write protected folders inside your own homedir. You can grant rights as any user you want simply by entering that username (it's automatically populated with the current user)
The only thing Windows does differently here is that it provides a silent, non-admin workaround (filesystem and registry virtualization) for buggy applications, which works, but which can be problematic in other ways. Most notably, while OS-level workarounds are a clear win for older and otherwise unsupported software, they reduce motivation for developers to fix bugs in current software papered over by Windows' workarounds.
Windows Phone 8 vs Android/iOS would be a fair comparison. Because WP8 has undergone the same treatment and has been optimised for high battery efficiencies whilst still retaining the Windows NT kernel at its core.
WP8 has always had support for high-resolution displays and Nokia et al have not had issues with it. WP7.x was a different story but that was a different OS kernel (WinCE not NT). During this time Microsoft was busy creating a minimised edition of the Win8 kernel that would be suitable for use on a smartphone, much like the work Apple did to OSX in order to create iOS.
Search in a page with plenty of marketing blurb for "mobile". Zero results.
So since they aren't marketing it as how you perceive perhaps you should cut them some slack.
ReFS would be a better example. And that's just another example of how Microsoft is keeping their core technology up to date. Apple meanwhile is still using HFS+ which even Linus Torvalds called out as being utterly crap.
This can be seen by the fact that Microsoft has never shied away from improving their OS kernel. Recent stand-out improvements such as ASLR, UAC, TRIM support for SSDs, timer coalescing etc. These are all things that OSX took years to get after Windows got them.