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The tricky bit from a user interface standpoint is selecting an exact tag rather than the hierarchy, such as if I wanted to look at Mythopoeic Fantasy Award winners without seeing the finalists. Tags in a hierarchy might be rendered with an accompanying glyph that is a link to see the exact tag rather than the hierarchy.
I use tags like like "Nobel Prize winner" and "Nobel Prize nominee", or "Newbery Medal" and "Newbery Honor" and then just use a tag search of "Pulitzer*" or "Newbery*" to bring up books in both categories.
If you make the tag specific, then using a wildcard allows you to bring up the more general group all at once.
Perhaps not as elegant, but it works for me.
@wildcards: that's a marginally acceptable solution, but I sure hate having to tag a book with both taiji and martial arts, or programming and java, etc.
Ideally we could all contribute to an LT ontology and make it easier to use other people's tags, but this rarely seems to work in "real life." :)
What you are wanting is really not tagging so much as it is a user-modified taxonomy/ontology. THAT is probably much more likely, and could live side-by-side in rainbowed harmony with tags.
As you said though....getting an open taxonomy to work is always a bit of a problem. </understatement>