I've had a look at a number of photo sets in Flickr to see how tagging was used. Pretty sloppy on the whole, I think. As far from LCSH headings as you can get. A number of contributors made up excessively long lists of tags which they added to each photo in their set whether the tags were relevant to the content or not. Others chose very obscure, made-up words to tag their photos - a private coding system? As a mechanism for describing photos tagging seems only to add extra serendipity to an already very serendipitous environment.
This exercise resurrected a memory of an exhibit I saw many years ago at Sydney's Museum of Contemporary Art. As I recollect, it consisted of approximately 10 copies of the same A4 sized photograph. Below each was an A4 sheet containing a minutely detailed description of the photograph. Each description focused on a particular viewpoint ranging from the wholly objective to the purely subjective. These are roughly the areas of description covered:
1) Physical - spacial dimensions
2) Chemical analysis of the paper and emulsion
3) Technical - how the photograph was made
4) Economic - commercial product
5) Historic - when it was made and its provenance
6) Thematic - style of composition
7) Sociological - its meaning in the society
8) Cultural - aesthetic properties
9) Psychological - emotional impact of the image
10) Philosophical - existential meaning - which would be totally unique and subjective for every viewer.
In case you are wondering what it was: a monochromatic sepia-toned commercial print made in the late 1880s; a figure study, not intended to be shown in polite company but quite tame by today's standards!
As an example of descriptive cataloguing, I don't think I have ever seen anything to match it. And yet it showed very clearly the variations of description that can be applied to just one photograph and how those will ultimately vary from one viewer to another.
So what is the value of tagging or folksonomies when there is unlikely to ever be any agreement on what tags are meaningful for all? There is certainly no thesaurus in evidence. I found an astonishing number of photos of groups of people partying when I had searched only for photos with tags that were place names. From looking at any of these photos you could never guess that the party was in Melbourne, Sydney, New York or Paris.
I am open to arguments for tagging, seeing as tags are popping up in all kinds of places - I've seen them recently on Amazon.com and they have been used in Fiction Connection (not very usefully in my experience), so we better get used to it. But at the moment, I'm not convinced.