Every time I read an article about attempts at non-hierarchical filesystems, I try to figure out how I'd take the huge piles of stuff I generate when I'm drawing (and publishing) a graphic novel and reorganize it under tags. It's never pretty.
Like, okay, sure, I tag everything with the name of the project, that's a no-brainer. But if I just do that then I get the hundreds of files I generate (one per page) mixed up with everything else - web-res renderings of each page, model sheets (and their source files), promotional material, stuff sent to publishers to try and convince them to deal with that part of the process, and the huge mass of files I generate for each book I print (which can be more than one for a single multi-year project): source files tweaked for print, print-res renderings thereof, files for the kickstarter for each book... So I tag all of these attributes too, and imagining putting all these tags on a file as I save it sure is a lot of fun, even if I imagine some sort of save requestor that keeps a list of all my previously-used tags, including ways to filter those - I don't care about any of the tags attached to my music collection or my collection of cartoon porn or my programming projects when I'm working on my comics projects, for instance, so I'd want to quickly narrow it down to just tags found in my art projects, and...
Ultimately it just starts to look like a hierarchical structure in my mind, except for the fact that I'm interacting with it by some kind of tag-filtering file browser on top of a huge filesystem that mixes everything together in a non-human-browseable structure.