IMO, the problem from a productivity and health and wellness standpoint isn't too much email, but too much of the wrong kind of email. I'd find my email load tolerable if it was only about stuff that I would find immediately useful.
The extremities of the email load bell curve are easily tackled. I can very successfully ignore or attend to the least and most important emails I receive - i.e. email sent to lists I don't care about which I filter and ignore until I hit a slow weekend, or email sent explicitly to me that I tag and read many times a day.
It's the stuff in the middle that I can neither keep up with nor ignore because they are occasionally useful that annoy me the most.
The email that falls in the middle of my bell curve bucket is usually sent to many people, and it would be trivial to apply the social voting concepts made popular by sites like del.icio.us and digg to these emails if everyone used an extensible email client like Gmail.
Thread count is a useful metric on occasion, but also inaccurate when it comes to predicting the importance of an email sent to an announce only list or where there's an email thread that goes on for ever because it's highly interesting to a small subset of the list.
As an aside you'll get email Karma snobbery if your emails are consistently voted as interesting.
Someone at work pointed out that newsgroups have always had scoring features. It's a shame newsgroups aren't used much anymore.
P.S. I get about 1500-2000 emails a day