This is actually something I've been wanting for a long, long time. Obviously I'm not planning on using this for / or anything system-related, but for my media archives &c it's perfect. Taking movies for example, I've found most people I know think an alphabetical approach is the way to go. This is useless, however, if I don't know what I want to watch. With the tags I can easily take tag in genres, directors, actors, etc and have my filesystem help me choose a film.
There is on major thing that I've considered would be an awesome addition to a system like this: automatic tagging from the metadata. It'd be awesome if my movies automatically got tagged by, say, resolution and length, so I wouldn't have to bother with such things.