Talking to the birthday boy via email, trying to track some conversational metadata, when I say, "I'm gonna start tagging my gmail," you know like body tags -- maybe, say, a couple tags per email or per thread -- that will pull very specific info when I search by my tags. Subject lines shouldn't be tags. They are a synopsis for the recipient. Tags are all about me. so if you see something like [[weekly status 11/13]] at the bottom of an email that isn't about weekly status at all, but maybe that's how I want to find it later, well then you'll know I'm tagging our thread.
Gmail thread tags, exchange tags, something to it?