I do see your point... but I disagree with this. Architecture / OS mechanics aren't the only place you can have a revolution. Windows 95 was a revolution in the human interface of Windows that led down the road they've followed ever since in terms of HI.
Likewise, OS X, which at debut was in many ways just a new windows manager on top of the same old Unix, was still a human interface revolution -- Apple in one fell swoop took Unix and created the first system on top of it that lots of typical home users could actually not only use but manage / self-administrate. Even a decade later, Linux is struggling to try to replicate this aspect of Apple's success.
HI is *important* -- it's not just a surface characteristic.
You will get no argument from this Macintosh user of nearly 20 years of the importance of HI. However, HI is not everything. Microsoft used the same GUI for its DOS-based Windows like Win95 and Win98 as it used for its NT-based Windows like Win2000. I would argue that Win98 was the best DOS-based version of Windows ever released. However, it was still DOS-based. Even though Windows 2000, XP, and Vista share similar HI with Win95/98/Me, they are far superior when the rubber hits the road. You saw this when the DOS-based paradigm collapsed under Windows Me.
As for your assertion that MacOS X is "just another window manager," I think that you go a bit too far. "Just another window manager" would imply that MacOS X is a GUI for the X Windowing System. It is not. MacOS X is both the windowing system and the windows manager. It is far richer than any combination of X Windows and windows manager on any other operating system.
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NT came about after IBM/Microsoft joined hands briefly to produce OS/2. Bill Gates was quoted as stating the future rides on OS/2. It was sad, really.. OS/2 Warp was for better, more stable, faster, more secure, than its contemporary, Windows 95. But IBM advertised via one Fiesta Bowl, while Microsoft advertised via *everything*. Win95 won, OS/2 died. And NT was born of it.
Discussions like this tend to ignore the relevant timeline. Microsoft and IBM began work on OS/2 in 1985. OS/2 was released in April 1987. Windows 95 was released in 1995 some eight years after the release of OS/2. Windows 95 did not kill OS/2. Bad but well-meaning decisions by IBM killed OS/2. Those decisions limited the adopted of the OS by a market that was largely satisfied with MS-DOS. When Microsoft saw that OS/2 was going nowhere, it stabbed it partner in the back and used its part of OS/2 to develop Windows NT.
I constantly hear OS/2 fans talk about how great OS/2 Warp was. OS/2 Warp was not the beginning of OS/2, it was the end of it. OS/2 Warp was released in 1994 seven years after the release of OS/2 1.0. Its claim to fame was that it ran Windows apps better than Windows. Can you imagine what would happen if Apple suddenly released a version of MacOS X that ran Windows apps out of the box? No more new MacOS X apps, that's what. OS/2 lingered for another seven years. In 2001, it succumbed to the inevitable.
I take a backseat to no one in my disdain for Microsoft. However, virtually every decision that contributed to the demise of OS/2 was made by IBM. They were borne out of the fact that IBM never grasped the fact that it had lost control of the personal computer market to the clones and that it could no longer dictate market trends. Today, IBM is completely out of the personal computer market.