The context is horrible (
4chan and 8chan, with a thread watcher
any regular booru
hentai foundry
pixiv
tumblr
deviant art
newgrounds
generic direct-image-linked gallery pages
), but the idea seems great.
Yeah, I think it represents an advancement in the state of the art of tag-based media organizational systems - with regular boorus (or any other tag-based online image gallery) for example, which all have a specific focus, you're not tagging the pictures that you have on your computer, you're tagging their pictures, so the database exists on their servers, doesn't work with any other gallery websites, and isn't related to whatever you have on your hard drive. Hydrus distributes tagging, so each owner of an image can tag it how they like, and then share those tags with other people who have an image of the same hash - the mapping (from hashes to tags) exists on your computer, can be easily shared and manipulated, and is directly related to files on your hard disk, not to mention that there are no gatekeepers and curators to decide if your files are worth hosting on their site.
It's a pretty incredible achievement - some things I would love to see added to it are some way of grouping mappings, or making particular sets of them special - like Gelbooru's mapping, and Danbooru's, and etc. This would open up the door to having transformations between sets of mappings (functors, you could perhaps call them?). For example, if you wanted to use Gelbooru's mappings (they are less strict than Danbooru) with Danbooru's tagging conventions, you could write some mapping of tags to tags (or, more like a mapping from mappings from hashes to tags, as the transformation of a single tag might depend on the presence of other tags), which could transform some of Gelbooru's tags into a set that follows the conventions followed on Danbooru.
Another important open problem seems to be that cryptographic hashing as the only equality metric for files is clearly too strict - thumbnails, minor image transformations (like PNG -> JPG), watermarking, etc, all appear as "different files" to the tagging system. I would love for there to be some way of equating files modulo some other function, like for images there is color histograms, keypoint matching, phash, etc.
I had never heard of or knew about the existence of any "booru" (this term is totally new to me, the only thing I know is that 4chan is disgusting).
I've taken a look and it's actually amazing what they can do with tags. I'll try hydrus with my personal files (photos, text files, audio recordings), but I imagine the problem will be that they're not all on the same computer at the same time[1], so maybe some glue will have to be made, or perhaps the library will have to be abstracted so it supports just hashes and also not only images.
The question of where does hydrus store its tags database is also worth making.
Resembles what git-annex tagging can do.