For a while now I've started using GMail's "Archive" button aggressively on my inbox. The end result has been that from thousands of emails, I now have 0 (zero) ! Everything is archived.
When I get a new email, it sits in the Inbox until it is resolved (ie. I reply or read it). Then it's instantly archived. Out of sight, out of mind.
I've found that this technique greatly reduces the information overload coming from emails. With a full inbox that was also showing snippets of the message (ie. small previews), every time I looked at my inbox I had some information to process. Like: oh, look, that one is starred, I wonder when they'll reply or hm, it's been quite some time since I've got an email from X as the name is on the bottom on the inbox, etc. etc.
Basically a full inbox sends you some information even when no unread emails exist. It's also quite a bad way to "search" for email. I used to manually look for some subject and/or sender in order to hit reply. Now I just use GMail's search.
I remember about some TED video where the host said something like our brain likes new information, we have an addiction for new stuff. Which is exactly what email feeds. It feeds our addiction for new things, even by just having a full list of previously received emails. I also assume that's why sites like Slashdot, Digg and Reddit are quite popular: they feed us new, easy to process, information. Imagine brain junk-food if you will or the Internet-equivalent of too much TV will rot your brain.
Related to this need to always get new stuff, I find it interesting the way Google handles this. When your inbox is empty, you get this message: No new mail! Want to read updates from your favorite sites? Try Google Reader (with a link to google reader).
So what Google is doing here is proving us what we have become used to. Not enough interruptions, not enough new stuff from email? Why gee, why don't you try this other source of new things: Google Reader. Come on, get a quick fix !