A few things I can see off the top of my head that would need to be solved for tag filesystems to be able to take off:

1. Inertia. Most software assumes hierarchical filesystems, and assumes it can control some portion of that hierarchy. This includes things ranging from search paths for various things ($PATH for binaries, library paths, etc), temporary files, preferences files for applications, assumptions made about hierarchical filesystems in archive formats like ZIP and TAR, etc.

2. Permissions. With a hierarchical filesystem, you can apply permissions on higher levels of the hierarchy to control access to lower levels, and you can have various forms of permission inheritance to control permissions on new files. Need a design for how to do that on tag filesystems.

3. Mounting. Filesystems come and go; some are on your OS drive, some are on removable media, some are network filesystems. Hierarchical filesystems means that each one has a single root, and it's easy to tell where the boundaries are.

4. Tagging taxonomy. What kinds of tags do you use? What happens if you mount a filesystem in which someone else used a different tagging taxonomy than you used? Who controls different parts of the tag space? What happens if you import an archive of material which uses a different tagging scheme than you use?

5. Projects. How do you group files of different types with different tags into discrete projects? How do you bundle related files together? How often would you want to see files by arbitrary tag lumped together, rather than looking in particular projects that have a pre-defined structure?

6. UI. How do you browse tags? How do you refine down? In many cases, rather than a general purpose tags based interface, you actually want media-specific browsers, like ones specialized for music which let you browse by artist, album, playlist, etc, or photo galleries that can show you previews of the photos, or video browsers which can show projects, bins, and sequences (for video editing), or IDEs which can either show you file hierarchy or allow you to browse by class, function, etc.

7. And finally, why is a tag-based filesystem necessary for this? What is wrong with the current approach, in which there are special purpose applications which can index, tag, and display certain types of media in certain ways? For instance, you can use your text editor or IDE to navigate among files within development projects, iTunes or Play Music or whatever to browse your music, Lightroom or Darktable or iPhoto or Google Photos to manage your pictures, iMovie or Final Cut or Premiere or Avid for browsing and managing your video, and so on. They all frequently have some way of tagging files, but also have specialized UIs for browsing the specific types of files they are defined for without having to do explicit tagging, and the actual files are just stored on a normal hierarchical filesystem.

There are a couple of good thoughts in the original post, but a lot is handwaved away, such as mutability of files, which is an incredibly important use case, for a huge amount of what files are used for today. Lots of people have brought up the idea of making tag based or database based filesystems, such as the failed WinFS effort (https://en.wikipedia.org/wiki/WinFS), but it's actually a pretty big problem to solve.