It has occurred to me that some kind of basic tagging would be useful here, but some of the uses I can imagine for tagging could be covered by other means. For example, rather than tagging threads, the thread titles (i.e. the first post title) could be edited with standardised labels, in order to differentiate, for example, between the request thread for a coding snack and its support thread, or its update thread, if these are different; or to label threads that discuss only one specific program.
As it is, even though certain areas of the forum are well organised and threads generally start in the right place, older threads - or threads that have changed their character - can sometimes end up mis-labelled. The effect of this is that even with a good search string, it's not always clear from the search results which thread is relevant. For example, in the Coding Snacks board, ideas that have come to fruition and have sprouted fully-fledged, named applications (mixing my flora and fauna metaphors there) are still to be found in threads titled "IDEA: description of idea". It would tidy things up a lot to rename these threads with something like "DONE:" and the application/script name.
I do think, however, back on the subject of tags, that whatever tagging system is introduced needs to be integrated into the search function. It would not seem useful to have to search either tags or by string. I'd like to be able to search for the title of a piece of software within the tag "review" or "recommendation", for example. (I can therefore imagine it being more useful to tag posts than threads.)