This reminds me of a talk from Clay Shirky about categorization and general ontology. It's interesting to read in hindsight, because it's from when recommendation algorithms were in their infancy.
> This is what we're starting to see with del.icio.us, with Flickr, with
systems that are allowing for and aggregating tags. The signal benefit of these systems is that they don't
recreate the structured, hierarchical categorization so often forced onto us by our physical systems.
Instead, we're dealing with a significant break -- by letting users tag URLs and then aggregating those
tags, we're going to be able to build alternate organizational systems, systems that, like the Web itself,
do a better job of letting individuals create value for one another, often without realizing it.
Warning PDF: https://ia800203.us.archive.org/10/items/Ontology_is_Overrat...
> This is what we're starting to see with del.icio.us, with Flickr, with systems that are allowing for and aggregating tags. The signal benefit of these systems is that they don't recreate the structured, hierarchical categorization so often forced onto us by our physical systems. Instead, we're dealing with a significant break -- by letting users tag URLs and then aggregating those tags, we're going to be able to build alternate organizational systems, systems that, like the Web itself, do a better job of letting individuals create value for one another, often without realizing it.