IMHO the big outstanding problems with email are spam and the difficulty of using public-key cryptography (both key distribution and mail client support). Writ

IMHO the big outstanding problems with email are spam and the difficulty of using public-key cryptography (both key distribution and mail client support). Write a dead-sexy mail client that makes the crypto completely invisible and uses either a public key directory or some kind of OAuth-ish thing, and you'd solve the latter. Better, if you made finding someone's public key take time and/or money, and simply refused to deliver unsigned mail, you'd take a big step toward killing spam.