The single, killer feature I'm looking for in a document management system (besides collaborative environment that we're used to from gdocs) is a way to stamp versions and have those be reviewed independently, with git like diffs across them.
Google Docs actually has this and hides it behind terrible UI/UX. You can "Name this version" of a doc, and there's a separate page to view versions (from which you can name versions as well).
The diffing isn't there, or at least not to the degree that code review tools offer.
I'm not sure the feature has evolved in years either. Definitely feels like one of those things a Google engineer threw into production one day, and it's never been considered again.
Do you mean document control, or diff on text contents?
For plain text, diff is do-able, but I don't know if comparing two PDFs can involve a detailed "diff" vs. a checksum, since the text could be the same but there's a change in layout, an image, etc.
For official documents, you want more than just change tracking. You also want formal approvals and per-document versioning, and repository-wide tags and Acked-By: lines just don't cut it.
Think gerrit for docs.