I actually use tags as directories in GMail. I can't find shit otherwise. (Tangent: just how shitty is the search in Outlook? I can never find anything unless labeled in advance.)

I agree for the apps. But then they shouldn't deal with files anyway. They should just access certain directories as permitted by the system, and those should also be exposed to the user.

Hard disagree on the documents (or anything else, really). One ends up emulating folders using tags anyway, and there's no real way of doing it in a platform-independent way. Also, searching can be very annoying in many cases. For my research, I end up working with the same files for a few weeks straight. It's much better if they're in a folder, rather than searching them every time.

You do you, though.