it'd be cool to see an information theoretical explanation of the complexity of hierarchical -vs- non-hierarchical file systems, and why human beings have such an initially easy time with hierarchies.

there's the obvious inheritance factor: the parent of a child in a hierarchy can provide a _heuristic_ about properties of the child. you sort of get this with a tagging system, but the inheritance can really only go one level (unless you make a hierarchy of tags, which brings you back to tree structures)