Yes, I've been exited about the idea of 'tagging filesystems' for a long time. The more I think about it, the more tags seems to me to be the best answer for the question of how to categorize the oodles of information we create for ourselves; much stronger than hierachial ordering. I would love for tags to be a natural property of a filesystem, and where that would automagically work with all FS tools, both high- and lowlevel. And yes, easy access to tag, and find by tags, in file-dialogs etc.
Maybe someone has already made this, and I'd love to get some pointers.