> "My most important point about "folders" is just that we should avoid referring to these new systems as such."

Fair enough and I absolutely agree. Though be prepared for users to refer to them as such.

And as to tag hierarchies - that's where things have been going. At least, those things I follow (DMSs, CMSs-not-named-Wordpress). It also addresses the concern raised upthread of "Company A\Secrets\Company B vs Company B\Secrets\Company A". With hierarchical tags, this isn't remotely a problem.