I think that the source and context of each tag are going to be major factors in determining what’s important in one of these systems. By “source”, I mean who said it and when they said it.

Let’s keep in mind that even though Flickr’s implementation of tagging is still in its infancy and lacking in some ways, tagging itself isn’t inherently flawed for those same reasons.

A tag is just a user’s way of saying, “link this photo’s to this word.” It’s a simple, direct way for users to build a network without having to learn some complicated syntax or GUI.

I agree that a master scheme is not a good idea, and I like the vibe behind the folksomies idea as well. And I think the more you scale, the more value can be derived from messy logic (look at PageRank, right?!). On the other hand, as amazing as Flickr is, has it gotten to the point of being the target of serious spammers yet? I’d be very interested in what you have done to stop it, if you can generalize…

However, the idea of tags (again, in Flickr’s case) is first and foremost for the benefit of the individual user, in organizing their own photos. In this regard, it works very well. The fact that it makes an interesting global collection and enables spontaneous creation of and participation in an endless number of interesting projects is a happy accident.

The global collection is, of course, quite incomplete and inconsistent – but who cares? It might irk you that there, say, are photos of the Parthenon that aren’t tagged with Athens, but so what? Lots of things irk me too. It doesn’t impede their use or efficacy.

Finally, a lot of the things you brought up about disambiguation, gaps in relatedness, alternate spellings, homographs, singular/plural forms, etc., have some interesting solutions that will be rolled out in the fullness of time

(And yes, there are plenty of ways to take care of spamming. See if you can find some – you might, but given a few million visitors monthly we do pretty well.)

+ Spam. What happens as Flickr and other services grow larger to keep the “tag spam” out? Is something in place from keeping spammers from mistagging images into popular categories?

+ What do tags mean?
Examples:
Old
Old what? Btw, I’m noticing pictures of newborns. What’s old about them?
Life
What does this word mean as it relates to images? Where’s the definition? Who decides?
+ Treo
Images taken by a Treo camera or images of Treos? Same thing with blogs.
+ Wonderful
+ Soda and Pop
They’re related. How? Pop, could also mean pictures of dad!
+ Does Apple mean the food or Mr. Jobs company?
+ Mom and mother.
What’s the difference. When I looked at each page today, one wasn’t listed as a related item for the other. Why not?

+ Spelling.
++ Someone looking at colorful might also want colourful.

+Pluralization
++ Differences between building and buildings? Many of the images with the tag “buildings” have images of just one building.

+ Comprehensive searching. For example, if
I search with two terms (bridges california) I’m going to miss all of these images of the Golden Gate Bridge.