Here is an interesting argument to the effect that tags aren't really helpful in classifying things. It focuses on journalistic articles, but seems to me valid for almost any area of knowledge: "Tags are not the right way of disclosing the rich interconnections between news articles and not the right way of packing them together."
They "are labels without any context. Tags are vague, and it’s difficult to tag content consistently."
The author, Stijn Debrouwere, argues that controlled vocabularies and relationships are the answer: "We need to re-engineer tags so that they’ll allow us to represent the rich relationships between our content and the things that content talks about." Relationships or connections are more important to the way we think about the world and our writing about it than tags suggest.
I need to think more about some of the most basic standard relationships of my stuff in Connectedtext. This article is a good starting point!