dystopia and utopia are infrequently-used tags here. Though I think they would be very useful tags at a story or pool level, they seem to be poorly used on an individual post level. Only one of the three uses of utopia is tagged on a post that explicitly describes the setting as such; the others appear to be tagged based on a subjective evaluation of what the tagger considers a utopia. dystopia is used more frequently, but almost all the posts seem to be tagged based on TWYK rather than anything in the post itself.
There's also dystopic and dystopian tags, which have similar problems.