[MetaCosm]

While we are getting good at proof-of-work stuff -- we aren't great a proof-of-power.

Meaning, it would be easy for me to emulate 10,000 slow computers with one powerful one, doing 2 minutes of work 10,000x parallel (claiming each of these computers is very weak) and that completely bypasses the "beautiful" solution.

Sending BTC works because it isn't gameable (at this point). I like the idea of a ฿0.0001 (or even ฿0.001, 20 cents) fee for each email sent to me. It is a little over 2 cents US -- would absolutely stop spammers (most it cost prohibitive for them), but hopefully would not stop normal users.