Tags are a really useful feature, but I think we could use some clarity about how tags ought to be used. For example, I've seen some posts with lots of irrelevant tags, and I think that tag spam detracts from the usefulness of the tagging system. What norms about tagging posts should we have?
I can't think of any posts I've seen that have a lot of irrelevant tags, but I could easily have seen some of them before they were tagged. Are there any examples you'd like to point out? (Though I do think JP's answer outlines the best response to these tags.)