Regarding Mutuality: I can see where even the invitation to a debate could become a means of uninvited aggression. E.g., Person A makes a comment on Facebook, Person B calls Person A an idiot prepares a debate and links them to it as a challenge. The implication being, if you decline, you forfeit.
Obviously such behaviour would be outside of the site's control, but it could be heavily mitigated by social cues from the UI and by the influence of the site's moderators.