One weird content tagging system I recall was Amazon's "Amapedia" (
https://en.wikipedia.org/wiki/Amapedia). It was a product wiki, a way for people to curate information of all sorts about Amazon products. It allowed each product to be arbitrarily tagged. It was short-lived, failed, and abandoned, for all of the reasons you'd immediately expect.
What was neat about it was that it must have involved someone a little too interested in set theory. A product was an article, and a product could have tags, but tags were themselves articles, and so tags could also have tags, and those tags could also belong to tags, etc.
The whole system was focused on these tags. If you wanted to compare two products, you'd compare the pages, and the comparison would focus on the differences in the tags of the two pages. Tags could have values, too, so products could have a "RAM" tag, and each RAM tag would have an associated value for that page, but the RAM page itself would have general information about RAM as a concept (which would probably have tags itself...). Searching worked the same way. You could search for pages with certain tags or tags whose values were greater/less/equal to whatever values.
Anyway, it was a fun and interesting way to do content tagging that did not work out.