While I think the above are sensible and necessary starting points, there’s another way to do this, through ‘views’ of all the data we have that can leverage the semantics and other clues such as folder location (music v pictures v documents etc), access control (public v private v group v individual-x), links (related to x) and of course the resource name and content.
Essentially views that can be standard, or customised, or which are really just a nice UI for a search engine tailored for personal data stores, but which could of course reach out from pod to pod.
I don’t think this negates the value of a sensible folder structure, but I can see a decent search-view solution becoming many people’s primary UI (it’s an app!).