I'd like to be able to write a comment about why I saved a particular email. Those comments could display along with the other selected fields (perhaps not for the inbox folder). I'd mostly use it for newsletters where I'd point out such things as "Nice Information about this or that," or "follow-up next month and ask about..."
When saving an email from the inbox, you could have a drop-down that includes "Move to" and "Move to with Comment."