Is there something I'm missing/something specific you're talking about?
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.
NeXTSTEP (carried on with OS X), NeWS, Irix are kind the exception on UNIX land.
There is a vertical integration from kernel to application programming and user desktop, alongside its hardware, to provide an unique experience.
In what hardware can do, what programming languages are the official one, THE framework to do XYZ.
Not a mismatch of pieces that often we need to break a corner so that they barely fit with each other.