Re: NeXTSTEP (@Mage)
Modern OS X uses, effectively, Display PDF. PDF is the output of a PostScript program. Apple's Core Graphics is all the same primitives, fill modes, etc as PostScript without the PostScript interpreter. So the two have both solved the same problem in the same way.
This actually turns out to be a pretty good idea: that's why Apple's text looks like printed text, using classic printed fonts like Helvetica, and Microsoft have had to commission their own custom fonts like Calibri that are designed around their idiosyncratic ideas about typography just so that the aggressive hinting, lack of pair kerning, etc, won't look quite so retro.
So what else did NextStep do that's interesting?
It learnt the Xerox Smalltalk lesson — that full object oriented, dynamic typed languages are a great match for UI work — but adapted the language so that it's compiled, not interpreted, and can link directly to the C libraries that were otherwise industry standard. That's Objective-C. It's just as happy talking with C++ nowadays, of course. The language and the framework are why the web was first developed on NextStep, why Doom was mostly developed on NextStep, etc.
It swept aside all the nonsense with application installers by introducing the application bundle. The application doesn't just look like a single icon in the Finder, it looks like one on disk too. Dragging it to the trash genuinely is an elemental file operation, not something that someone has hacked in as a special case. (aside: RISC OS did more or less the same thing at more or less the same time, as well as the dock and a focus on proper typography; all coincidence, apparently)
It introduced the fully compositing window manager. Consider where Windows was up to and including XP: preemptive multitasking, protected memory. So it doesn't affect the wider system if an individual app hangs or flips out, right? The answer is: only if you don't care whether the screen is painted properly.
File associations are handled by metadata, not as an exercise in string matching. If I want .doc to associate with Pages by default but have a few that render incorrectly and should be opened with full-on Word, I can set those to open with Word while leaving the rest alone. This becomes a property of the file and goes wherever the file goes. It is not a hack someone added into the Finder.
It was the first graphical environment with system-wide scripting. It was designed from day one to be architecture agnostic, supporting fat binaries. It beat OS/2 to the punch on both of these things.
Beyond that the big wins are really in the frameworks themselves. Pervasive rich text, system-wide spell checking, a system-wide encrypted store for passwords, etc.
So none of those is individually a massive leap (though it depends what you compare it to; if it's only commercial competitors then Objective-C would count, as someone finally realised what Xerox had pioneered under the hood) but I'd agree that NextStep was a decade ahead in the '80s based on the combination of technologies.
Though, yeah, then they decided to price it beyond any sense and predicate the machines on a dodgy media format. I guess Jobs learnt how to price things for optimum profits by reeling in from the far end.