The filesystem on desktop computers is a huge skeuomorph.
Files exist in one place and there's only one path to a given folder. Yes, symlinks, aliases, and shortcuts exist, but only as afterthoughts. Even tagging files is only relevant for within-folder sorting or search parameters.
The ability for things to be accessible from more than one place at once is effortless for modern computers. That should be equally effortless for users to take advantage of.
Many of our files already behave in this way through abstraction layers like iTunes, Lightroom, iPhoto, and the Recents menu in many apps.
These are all siloed efforts to better organize how we access specific kinds of files. And you know what? They're not bad at it.
Lightroom has collections of user selected photos as well as rich keyboarding tools.
iTunes has albums, user defined playlists, artists, as well as rich metadata searching to get the songs you want.
These are for individual document types rather than entire filesystems.
What if the filesystem itself worked this way? Users could define their album for a class, save a search for things (documents, photos, etc) containing the words "New York City" or created between the dates December 17th 2011 and January 4th 2011.
These capabilities exist from a technical perspective, but they aren't easily usable.
How about it guys, Dropbox, Google Apple? I could use a file system that wasn't made in the 80s.