Now I need to charge users before I let them sign up for an account, because if I send them a confirmation mail when they sign up, and then a couple password resets over the years and I've got more than a handful of users, it's going to be real money. Which isn't that hard, but users don't really want to pay $1 to join a forum or whatever.
The bigger problem becomes how do you actually tape a penny to an email? There's a lot of diversity in email providers (even if there's a lot of concentration at the top), and international payment flows are extra fun. Plus, a penny USD is a significant amount of money to people in many less wealthy nations.
And the whole thing is very decentralized, so good luck with adoption.