I wasn't alive at the time NeXTSTEP was a thing, but I did look at a demo[0] to figure out what you were talking about (i love building/tinkering with window managers); it just looks like a regular old window manager?

Is there something I'm missing/something specific you're talking about?

[0] https://www.youtube.com/watch?v=rf5o5liZxnA

NeXTSTEP was everything from the OS to the user experience and everything inbetween.

I'd say there were 3 distinct abstractions within NextSTEP: - The microkernel / OS (Mach / BSD) (for the hardware) - The Objective C based SDK - The User experience (not just window manager, but largely the window manager)

The SDK is what is still arguably the most highly regarded part of NeXTSTEP even today. That aside, at the time nothing else was so well polished and integrated on almost every level.