*bulk* e-mail. Whatever we may think about the content, one's and two's of e-mail just aren't anything more than annoyances, if they're even that -- they're just the price of having a mailbox: sometimes you'll just have to cope with mail you don't want. Thus one requisite for censure is that the mailer does *bulk* mailing.
However, that's not enough. The net is about communication -- and people will regulate their own reception of mail, one way or another, *provided* that they're given the opportunity. *Unsolicited* e-mail bypasses the opportunity and bulk unsolicited e-mail has the real capacity of making people's use of e-mail impossibly expensive, in terms of money and time.
Thus, I would say, that we must focus on *bulk* *unsolicited* e-mailing; other factors, such as content, are secondary.
And, just to point out that the flyer analogy is not irrelevant, do we *really* want to, by restricting ourselves to *commercial* mail, effectively sanction the Crusader spammage? Or the rape and snuff spammage? Or, what happens when the Church of Scientology decides to exploit that medium....?