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.