I remember a lot that was "good or unique":

Prime was a cheap Multics complete with processor security rings, gate segments for user programs to call into the OS, an hierarchical file system, and capabilities and attribute control list hierarchies based on the hierarchical file system. The file system used the relatively large physical record size of 4096 bytes, and the file system continued to work great as we ran hard disks for years without ever once doing anything like a decompression to get all the physical records of a file closer together on the disk for better performance.

For more performance, they had closely coupled systems.

The terminal I/O was super nice and easy to use, 8 bit ASCII at 9600 bits per second, easy DIY cabling, with easy to use DC1/DC3 XON/XOFF handshaking protocol that worked well with lots of ASCII devices,
e.g., a nice HP color plotter and the Xerox Diablo daisy wheel printers. Users were not locked into too expensive terminals but could do well with just dumb glass teletypes.

Their standard text editor had some nice ability to be programmed, and they had a nice interpretive scripting language. They had a plenty good enough word processing program well in the theme of the Bell Labs RUNOFF. Prime used that and daisy wheel printers to prepare their documentation.

It has been claimed that the Intel 286 architecture borrowed heavily from Prime's architecture.

On the first Prime I managed, I did a lot of applied math for US national security and also did the software and typing for my dissertation. One prof needed a little clarification in one paragraph, so on the weekend I typed again for a few minutes, had the daisy wheel printer retype the whole dissertation, and submitted the new version on Monday. That is the version that was accepted.

Our work for the Navy very much needed better word whacking: Once
we submitted a report from typewriters with so much correction fluid the Navy refused the report. As soon as people saw the file system, terminals, text editor, RUNOFF, and the daisy wheel printer, each of the secretaries begged for and got their own terminal, quickly got a little instruction, taught themselves, kicked their typewriters out the door, and became terrific with word processing. Suddenly the volume and quality of the word processing went through the roof.

We paid for the Prime with about 18 months of our earlier time sharing bills and got MUCH more computing than we ever had on time sharing.

The Prime I helped the B-school with did student work, faculty work, administrative work, alumni office work for fund raising from alumni, and word processing.

The alumni office did a little work with data base and sent boxes of personalized letters to alumni for fund raising. The little system the student assistants put together was copied by the central university computing group for use for the whole campus. Just from the alumni work, the system likely paid for itself in green dollars many times over.

We had a quite nice collection of scientific and engineering software, actually nicer than the IBM Watson lab at Yorktown Heights had when I later went there for work in an AI group.

The system was wildly successful for years.

To me, at the time, Primos was nicely better than Unix. Prime had a great opportunity to port Primos to the Intel 286/386 but didn't and missed out.

IIRC, the first computing Bloomberg used for his trader information terminals was from Prime.

Prime had a good thing going, but Wintel beat them but should not have been able to. Primos was far ahead of Windows for a long time and could have been ported to Intel long before, say, Windows NT or 2000. And I can believe that Primos was relatively secure.

IIRC, Primos ran in about 40 KB so would fit into even the 640 KB of the
first PCs and certainly would fit in the memories of the 286 and 386 systems that ran Windows 95, NT, 2000, etc.

When I was in grad school, I was reading the material from D. Knuth about his TeX mathematical typesetting system and hoped to get that on the Prime at the B-school. Well, one of our bright computer center guys got started on porting TeX to Prime. We met Knuth at a conference, and thanks to that guy our Prime became the official distribution point for TeX on Prime. The same guy also got us going on e-mail.

The B-school Prime was a good site and very successful for 10+ years.