Thanks for dropping by! I now realize that my reply on that thread wasn't exactly responding to that interpretation. However, your reasoning is part of why I have always been conservative about their implementation here. IMO tags work as a dimension of discovery or for editorialization (a la twitter hashtags), but that they break down when used to categorize content within a community for reasons that include the one you gave. Even so, simply having the option gives people the impression that they should be used for categorization. I think Twitter hashtags succeed in that they eat up the limited characters and have a transient nature by being embedded in the content. However, am I correct that the way you implemented them on Reddit was to alllow a post to exist in more than one subreddit at once? If so, that seems like a slightly different animal in that the communities were established, and the tags dipped the posts into these predefined spaces.