At that time "open" meant interopable rather than the current "open source software" meaning. For instance, the DBkit and EOF database software could talk to Sybase or Oracle or any other database for which there was a driver.
As a result we used NeXTstations for all our tasks, and really, that was the best computing environment I have used. As another poster says, we really haven't "budged the needle" since Nextstep v3.3 of the OS.
Later while working for another company I got the ability to talk to Keith Ohlfs who designed the UI and personally thanked him for it.