Tags or folders?
Bergman and co-authors explored this issue in the JASIST paper "Folder vs. tag preference in personal information management."
Tags are apparently preferred because they enable "multiple classification" and a lack of "hierarchical location" so that information doesn't get hidden.
The reminding function of information piles is an important concept. As long as information is on the desktop people know that they have to do something with it.
As an aside, how do we tag messages in email? Not easily, apparently. There are Outlook add-ons like Email Tags from Outlook4Business. It seems to work via an archiving process using Outlook categories.