The problem with metadata, as Armando clearly posited, is portability. Currently as it is Leopard has fantastic metadata support (they hired the BeOS file system designer after all!). There are some 120+ core metadata attributes for any file in my system, this is accessible from any app, or even the command-line. A daemon, mds (metadataserver), uses FSEvents notification so when any new file lands in the file system from anywhere, it scans the file and fills up as much metadata attributes as it can. Any developer can code a metadata importer to import their proprietary formats metadata in the same transparent way. All of that is immediately available via spotlight, or whatever UI you want to build round the core framework. And as a core part of the OS and file system (not some 3rd party hack on top), this service is available to all. Apple even have a way of saving this metadata on non-apple drives, where a sidecar file is generated (like XMP in some ways).
BUT - when I give my file to my colleague who is not using OS X, they can't use all that juicy metadata goodness. I and many others were hoping Vista would pick up the can from the crappy metadata support in XP, so that a bridge could be built. But alas.
Another problem is stasis. People are used to folders, they've twisted their mental models to traversing up and down hierarchies like an indecisive mountaineer; of arbitrarily having to classify something as "a" OR "b". And so though the technological base is finally there at least in one popular OS (BeOS not being so popular sadly
), adoption of it will take time, and people are wary of change. I want the future today goddamit! But until then clumsy stopgaps and programs like Noah will have to do.