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Your reply above was why I had belated moved my comment about NT/Unix to a separate reply from my IBM history reply (I hadn't realized you were already composing a response - sorry about that). I wanted to see a fuller discussion from you.
I've been doing some more browsing because that memory of some ribbing in the press for "unix borrowings" (whatever that means) is fairly firm. Based on your comments above and also a book review I found covering a book on the development of NT, I suspect the ribbing quality came from some rather public anomisity towards Unix by the NT team, but I only suspect. I have no memory of that either way and haven't read the book - just the "so there" quality in the press.
The comment about claiming A is like B because of superficial similarities rings a bell. While browsing around the web looking for where and what was behind whatever it was I remember being reported, I came across an article in InfoWorld way back when in 1987 arguing that *DOS* "borrowed" from Unix because it had pipes. InfoWorld, Aug 3, 1987:
from the start, DOS has been influenced by Unix. DOS 2.0 explicitly incorporated directory structures, pipes, redirection and other features that had long been hallmarks of other systems.
It may also be that those reporting on NT and OS/2 development falsely understood the built-in POSIX support as "Unix borrowings". According to Wikipedia,
Windows NT-based operating systems up to Windows 2000 had a POSIX layer built into the operating system, and UNIX Services for Windows provided a UNIX-like operating environment.
My only quibble with your post is this line: "There are obviously many things that every OS has to have in common."
They may have to, but by what definition? There are many things we now assume as must haves in OS's that were not part of some early operating systems - including multi-tasking. I agree that superficial borrowings of features are not the same as borrowing core internal architecture. On the other hand, the features that eventually became attractive enough to be "must-have" s in other systems, were built-in right from the start in *nix.
I do think we have to be careful about assuming that what now seems to us a no-brainer was always commonly accepted as so. The best innovations are tricky that way: once they are stated they seem so obvious we wonder why they are even innovative.