Not much, unfortunately. File system metadata query and responsiveness is something that I haven't seen matched still. The latter is an exhilarating feeling that I'll never forget. Also, the UI had a “fun” but still elegant feel to it that I was very fond of.
The below is based on vague memories from 20+ years ago...
Some examples:
Every email received was stored as a file. All metadata about the email (sender ,subject, headers, etc) were stored as attributes.
Tracker, the file manager application, could display any arbitrary attribute. I seem to remember that instead of an email application's window, my inbox was a simple Tracker window showing the specific attributes required to know about my emails.
My music collection was stored and displayed the same way.
Everything was searchable but also usable. File attributes weren't hidden behind CLI commands and flags like xattr, they were part of the GUI and every application using the SDK would know/understand them.
This was, at least in part, how Microsoft's "Internet News and Mail" worked. Not sure where it kept metadata, but the initial implementation seemed like an Windows Explorer extension, which was a very clever approach, IMHO.
I wish Gnome did an e-mail client like that, synchronizing a local folder with a remote IMAP mail store, or presenting a remote IMAP storage as a mounted folder under an IMAPFS sort of thing. Arguably, that's easier to do with POP3. Same thing with an outbox, where writing a properly formatted file would send an e-mail.
> File attributes weren't hidden behind CLI commands and flags like xattr
This is a design choice in the UI, not something intrinsic to the OS.
BeFS maintained indices of extended attributes; in other OSes indexing is done in userspace which is safer and more flexible but can introduce race conditions between filesystem operations (e.g. moving a file) and updating the index.
