I believe it would be a good idea to have a tag to list all the well-tagged posts.
That is, posts that we have taken the trouble to hunt down everything to tag as being in the picture. In other words, you could not easily add another existing tag to any of these posts, if you tried. (Maybe you could, but not very easily. All the possibilities of eyes - e.g., tsurime, etc. -, clothes, and so on should already have been explored enough.)
I believe post #113189 and post #1099536, with the current tags they have, would qualify. The former is a simple concept: a drawing of an empty library, so it is relatively easy to be satisfactorily tagged, once we know what are the necessary tags: no_humans, still_life, and so own. (it has also some tables and no_lineart, for example) The latter is well-tagged, too, despite having a lot of details concerning Izayoi Sakuya's posture, clothes, weapons, appearance and art.
To play it safe, the presence or absence of select controversial tags like truth and what, and whatever random underused recently-created tags we might have, would NOT count to qualify a post as being "well-tagged". I believe brown (and other color tags), epic and creepy (and other "feeling" tags) would be "less subjective" examples, but I would want to avoid problems with them, should they arise, so I guess it would be best simply not counting them for that purpose as well. (If there are other criteria for what counts as a well-tagged post, I'd like to hear them.)
At the moment, we have the pool #876 ("Tag PANIC!"), with a related purpose. Apparently, it is supposed to have all the posts with 75 tags or more. The description implies that the posts were throughly tagged (They use the words "hunting down everything to tag as being in the picture.", which I paraphrased above for my own purposes.), but that is not always the case, as some posts can be better tagged even after reaching the limit of 75 tags. Conversely, I would want to add the "well-tagged" tag to all the posts that qualify, including the ones so simple as to inevitably have less than 75 tags, like the library post I linked above.
For the moment, I populated the tag finished with approximately 125 posts I've found to be well-tagged, with those criteria in mind. A search for izayoi_sakuya finished is supposed to show all the "finished" posts of Izayoi Sakuya.
These posts would serve as examples for tagging other posts. If we are curious as to what are the right tags for Izayoi Sakuya clothes and hairstyle, we can see the list of izayoi_sakuya finished posts, to find maid_headdress, braid, and so on, then procceed to add these tags to other images of her, when they apply.
I've chosen "finished" as the tag name, based on translated and annotated, two other tags that qualify work done by Danbooru users on the post. In all cases, the practical details (exactly HOW can we make a post "finished" or "well-tagged" or "annotated" or "translated") are inevitably left to be explained on their respective wikis. Despite that, I naturally appreciate the fact that finished is meaningful on its own (as in "finished work"), like "annotated" and "translated".
A minor use of the tag finished: Obviously, searching for gentags:<20 yields a list of all the posts with less than 20 general tags - undoubtedly, most of which are simply poorly tagged posts in need of work. Differently, searching for finished gentags:<20 would not yield any undertagged post, by definition, so we only see actual examples of works so simple we actually could not find enough tags for them.
The tag finished could potentially be added to all the posts (much like translated is virtually everywhere), but I certainly don't see it as a bad thing: the more posts qualify for finished, the better. If all posts had that tag, it would mean that Danbooru is 100% well-tagged. (even if that's a form of unattainable perfection)
Sadly, the way I planned it, the tag finished is a bit subjective for not counting controversial tags like truth and recently created random ones. If it is unclear which posts deserve to be tagged finished, then it might be misused on posts that are too under-tagged, for some reason. Like all other tags, it may require some cleanup work every once in a while, in which case I would be glad to help.
Thankfully, one can search for finished order:change_desc to see the "finished" posts by the order in which their tags were last changed. Since, logically, finished posts aren't normally expected to ever change tags at all, (except when changing, say, translation_request for translated) the last change would be the addition of the "finished" tag; so, basically, we would see the posts by the order in which they gained the "finished" tag, which would help a lot to do any cleanup work and/or see how the tag is growing in use. Most other tag changes would be "anomalies" - for example, if someone added a tsurime tag that was forgotten even after the finished was added.
I apologize for taking so long to explain the idea.
If there are any problems I didn't foresee, alternative ideas, etc. (maybe a different name instead of finished?), please let me know. Thanks in advance.
Updated by jjj14