Insist that readers be who they are, and not attempt to pass themselves off as someone else. If you[r] site allows pseudonymous posting, insist that readers use a consistent handle or account name, and take whatever technical steps you can to keep people from posting under others' names.
Don't allow readers to mislead others about their identity, either. Warn readers against omitting information from their profiles or posts that would lead other readers to believe that they are someone other than who they are. Elected officials shouldn't be allowed to pretend that they are not when posting to a discussion about local politics, to use the Telegraph's example.
These recommendations are important from a practical, ethical perspective more so than from a legal perspective because CDA 230 (47 U.S.C. § 230(c)(1)) gives website operators immunity for publishing content submitted by others under most circumstances. From this practical, ethical perspective, however, I agree wholeheartedly with Niles. There is a difference between respecting and promoting a user's ability to engage in anonymous speech and allowing a user to mislead others and manipulate the tools put at his/her disposal. Perhaps a Term prohibiting impersonation would be hard to enforce (maybe not?) -- at the very least, a Term of this kind puts users on notice about what kind of community you want to create and makes a statement about engaging in speech and debate responsibly.