Technology Review has an article by Vipul Ved Prakash, founder of Cloudmark:
Here’s a list of three rules (created after the most important features of e-mail) that anti-spam software should strive to follow:
1) Ability to send and receive e-mail from a stranger. (Whitelisting, payment systems, and challenge/response break this rule.)
2) Ability to send and receive pseudo-anonymous e-mail. (Domain-based authentication breaks this rule.)
3) E-mail should be free. (Payment systems break this rule.)
If we can solve the spam problem while maintaining the three features of e-mail, it would be a much sweeter victory.