My parents (the only, well, “casual” users I know really well) have all their documents in one folder with no hierarchy whatsoever, any sort of library would improve their experience. There is enough metadata to make it useful, I think. There is the time of creation, the time a document was last edited, its name – that’s more or less just as much metadata as I use for browsing my photos. (I don’t spend much time organizing my photos and my photo library is not very powerful – I cannot, for example, only display all photos taken with a certain shutter speed. Despite that I would really never want to go back to organizing my photos in the filesystem.)

A sort of document library could be just as good or better than the filesystem for managing documents, I don’t think that you have to end up with a leaky abstraction. (Browsing documents on Google Docs, for example, doesn’t feel wrong but you are not using a filesystem to access your documents, you are using a sort of library view.)

I think it’s kind of astonishing that no one has really tried to do just that. There are half-hearted attempts but one of the things about abstractions is that they have to be good to be successful. It’s certainly not easy but I think it’s possible.