For starters having been written in C, after the Assembly and B rewrites, when we already had safer systems programming languages in the early 60's.
IBM for example did most of their RISC research with an OS written in PL/8, where they prototyped many ideas about modular compilers, a few of them visible in how LLVM works.
Then by being a plain CLI based OS versus what was being done at Xerox PARC with Xerox Star and Pilot (done in Mesa).
NeXTSTep was probably the only variant that got close of the Mac, Amiga and Atari multimedia capabilities, and even it wasn't a pure UNIX, rather used it as a means to bring applications into the NeXT world.
Many mainframe ideas about OS security are still not part of any UNIX clone, as another example.