I used BeOS as my main driver for about a year and a half, back in the day.

One thing that we take for granted now was it supported low-latency concurrency, so its user interface felt really, REALLY snappy. You could launch 4-5 different videos, in different windows all playing at once, and there was absolutely no jitter and lag. Unless you grew up with the user interfaces of the mid-90's (Windows 3.1/95, MacOS 7.6), you won't appreciate just how mind-boggling this was.

The other big feature that unfortunately hasn't been broadly adopted by other operating systems was the file system. I know this one pretty well because I actually wrote my own driver for it (as a high school's hobby project mind you, so probably extremely cringe if I saw the source code again). BeOS didn't have a hierarchical file system so much as a relational database with built-in support for showing nested relationships as folders, and support for multi-megabyte blob types for the actual file data. Emails, for example, were stored with each header value as a separate column in the email table. If you opened your folder of emails, you could sort by sender, time received, subject, etc. just from the finder window. Within the BeOS API, you could write complex joins on the filesystem, e.g. between your emails and your contact list (which was also stored as metadata IIRC).

dr_dank summed it up back in 2003 [0]:

"BeOS was demonstrated to me during my senior year of college. The guy giving the talk played upwards of two dozen mp3s, a dozen or so movie trailers, the GL teapot thing, etc. simultanously. None of the apps skipped a beat. Then, he pulled out the showstopper.

"He yanked the plug on the box.

"Within 20 seconds or so of restarting, the machine was chugging away with all of its media files in the place they were when they were halted, as if nothing had happened.

"Damn."

[0] https://slashdot.org/comments.pl?sid=66224&cid=6095472

From what I heard, the filesystem was very cool. You could attach arbitrary key-value pairs to files, and e.g. rather than store song metadata in ID3 tags or whatever, it was stored as filesystem metadata, making it searchable by any program without knowledge of a file's internals. As I recall reading, one could create a virtual folder that contained the results of a search, e.g. "all MP3 files where artist='Pink Floyd'" and use that as a playlist.

I have played around a bit with Haiku, but have not figured out how to do that so far.

if I recall, (and someone correct me if I'm wrong), it had deep multithreaded support/design, more so than any other desktop OS at the time, so it could run more simultaneous processes and apps w/o getting bogged down. at least I think there was a demo claiming this.