I love Unix.
It's my favorite OS.
And I like it for its fundamental process model.
That combined with stdin/out and pipes.
All stitched together with a process aware shell.
Lots (most) OSes had a process concept. But in Unix, they not only existed, they were everywhere, they were dynamic, and they were "cheap". They were user accessible. A process with its ubiquitous stdin/out interface gave us great composablilty. We can click the processes together like legos.
For example, VMS had processes. But after 4 years of using it, I never tossed processes around using it like I did on Unix. I never "shelled out" of an editor. I never & something into the background. Just never came up. One terminal, one process.
On Unix, however, oh yea. Pipe construct on the command line, bang out of the editor, :r! in vi. And the eco-system the was created out of this simple concept. The "Unix Way(tm)".
And anything was a process. A C program. A shell script. At this level, everything was a "system language".
Then, They (those Unix wizard folks) made networking a well behaved citizen in this process stdin/out world. `inetd` could turn ANYTHING (because everything had stdin/out) into a network server. This command is magic: `ls | cpio -ov | rsh otherhost cat > /dev/tape`
Does `ls` know anything about file archives? No. Does `cpio` know anything about networking, or tape drives? No. Heck, `cat` doesn't know anything about tape drives.
You just could not, back in the day, plumb processes and computers together trivially like you could with Unix. Humans could do this. They didn't need to be wizard status, "admins", guys in white coats locked in the raised floor rooms, huffing Halon on the side. Assuming you could grok the arcane syntaxes (which, absolutely, were legion), you could make Unix do amazing things.
This flexibility allowed me to make all sorts of Rube-Goldbergian constructs of data flows and processes. Unix has always been empowering, and not constraining, once you accept it for what it is.