Have followed all the links and updated the tags in my blog. I can see I had better read The cult of the amateur; from the review, I suspect I would have more to say about it under Thing 22, Wikis, than at the present point in the agenda.
The Haddon Library uses the Bliss Classification Scheme. If you are familiar with Bliss, you will know that it builds classmarks by combination, which can make them inordinately long. At the Haddon, our way round that is to limit the specificity of any given book's classmark, and/or the number of facets of the book's subject that the classmark may cover. If necessary, we include additional classmarks in the catalogue record; and the English-language translations of those main and added classmarks are in the catalogue record as well, functioning like Library of Congress Subject Headings.
It is quite a job to keep these translations consistent with one another and with current usage, both specialist and non-specialist, and I can see a lot of point, the more so in a subject library, in letting readers augment the catalogue record with tags of their own.