I am an active user on chemistry.SE and I've recently become interested in Earth Science and have started to participate over here as well. One of things I noticed whilst browsing around the site was the extremely large number of tags that we have, many of which are applied to very few questions. For comparison, on earthscience.SE there are 341 tags (including synonyms) and 1785 questions. Just under half of these tags are applied to five questions or fewer. On chemistry.SE we have 267 tags and 15064 questions with only 22 tags that have five or fewer questions.
Even on the first page of most popular tags there are several which seem like they could be merged or are poorly defined. For example:
earth-history seems very poorly defined. Most of the questions are about events in the Earth's past (which is a lot of Earth Science) but the tag guidance seems to be about the history of geology.
rocks seems like a very vague tag.
models is similarly vague.
Going to the end of the tag list there are a whole load of tags which are either complete junk, like local, or seem far too specific to be of much use.
On chemistry.SE we have had quite a lot of discussions about tags and come up with quite a comprehensive post which you can read here. In particular, there is a guide for whether or not a tag may be worth creating.
Proposal: We should have a meta discussion about what to do with tags on this site. I am in favour of having an event like this where we spend some time clearing out tags and organising questions more effectively.