The NeXT OS used Display PostScript, and I believe that is still the core of OS X Quartz, is it not?

Are you saying that there was not a good API for drawing on the display? You had to spit out raw PS?

It was more than that, you wrote your entire UI in NeWS's Postscript-with-knobs on, and communicated asynchronously with the server side of your app. Think of writing a desktop app as a tiny web-server with the UI in a browser, only all the UI is in PS rather than HTML/CSS/JavaScript.