NeXTSTEP/OPENSTEP was just great. MacOS X is obviously the successor to it and picks up a lot of what made it great, but it’s definitely bloated compared to what was.
The OPENSTEP API is published as a specification, which is fantastic because it means someone else could implement it (e.g. GNUStep), but since Objective-C was never formally specified that kinda leaves it dead in the water…
I used to rock a cube with a NextDimension board. It was wonderful. Having said that, I always preferred the Archaic Mac OS to NextStep, and I felt that Apple made basically all the wrong compromises when they were bought by Next.
NeXTSTEP really does run quite well on these, and NeXT’s almost seamless fat binary building means almost everything is available for PA-RISC.