Nice thing about X was that it would talk to remote displays. I still remember sitting
in the Pentagon demonstrating that the Suntools screen lock wasn't particularly
secure.
Then there was NeWS. This was Gosling's first attempt at a deployable language.
However PostScript (even with Owen Densmore's class extensions), while a reasonable
intermediary language is really sucky to actually develop. Java was a bit more refined.
Of course, lots of things either implement X under the native window system or backdoor X
with local extensions. We got around doing high frame rate image work on X via the
SharedMemoryExtension and the ability to flip buffers on the retrace interval (both
extensions, but commonly implemented by many servers).