The ease with which you could drop in new filesystem drivers is another one of those things that was great. Aside from the official FFS there's been a number of other filesystems even long after Commodore went under.

A terrible idea because they were backed into a corner. Had the third party producing CAOS delivered, there would have been no need for an insane timescale port of Tripos to become AmigaDOS. Then floppies wouldn't have got OFS which was a HDD filesystem hacked to fit in as little time as possible. dos.library would have escaped the horrible BCPL mucking about with BSTRs and BPTRs too.