I originally wrote this as a comment on the State of the Beta question, but decided to move it into its own thread once it got pretty long and detailed.
I have what I think is a problem, with no particular solutions in mind: tags are not very useful right now, compared to the other SE sites I use.
The core problem, as I see it, is that it's not clear what tags are for. Some are general topics like key-bindings. Others describe a relevant core feature like isearch. Others describe an action you're taking, like syntax-highlighting. Others categorize by Emacs package (org-mode) or external tool (git). Then we have some special ones, like [emacs-24.4] for version-specific questions.
The problem is that these categories behave differently—often orthogonally. And the result is that we have a very long tail of tags with very few questions (5-15), which are mostly not very useful. Moreover, since there are so many different types of tags which can all apply to a single question, I suspect many questions are missing relevant tags, leading to even more fragmentation.
This makes tags more difficult to use for both asking and answering questions.
For asking, it's difficult to figure out which tags are necessary. Unless you're in one of a few very clear-cut cases (org-mode), you have to guess something abstract—and since so many tags have a similarly small number of questions, it's hard to naturally tend towards the same tag other people are using. As it stands, there is no real pressure to consolidate on given tags, so the fragmentation problem isn't going to go away by itself.
On the flip side, as somebody who likes to answer questions, it's hard to find relevant ones. A lot of the more abstract tags don't work for me at all: seeing that a question is tagged syntax-highlighting doesn't tell me anything about what concrete knowledge is needed to answer it. More generally, I haven't found any good tags to follow, which is the main way I filter through to answerable questions on StackOverflow and Programmers.
As a concrete illustration of the problem, I've written 14 answers and 9 questions as of now, across 49 tags. Not a single one was ever repeated for more than one post, except for the one time when I answered my own question!
I think we need a holistic review of existing tags, with the aim to consolidate and pare them down enough to be more useful. But I don't have any ideas about how to do this that are specific enough for immediate action.