Better said, how much harder is to mantain a forum/message-board/community webiste with unauthenticated users and what would be the benefits and disadvantages in comparison to an account-based one?
What about letting unauthenticated users have at least some rights like posting, flagging or voting? Is that so much harder to implement in the matter of security?
The way I see it, it would be much better -and natural- to have an authentication process as an experience enhancer only (having more options and rights, no CAPTCHAs, etc.) and not as a requirement for participation (unless users save or share private information, of course). I don't know if this makes any sense at all.