Community Guidlines are based on Subjective concepts (Politeness, Decency, Civility..etc..)
Stopping determined violators (Spam, content, etc..) can be daunting.
Whether to Post or Pre moderate?
Managing real time member generated content is difficult (IRC chat, message board, uploded photos).
Pending Legislation may affect the sorts of content members may allow. Websites may be considered "secondary producers" of any content they host (See 2257).
User generated content can be linked to Revenue.
Social Approaches:
Be open to new approaches and feedback.
Clear and easy to find Rules of Conduct.
Having a path of appeal in regards to moderated content.
Open Communication with members regarding Procedures.
Giving moderation tools to users
User activated filters
Reputation Rating (users with low ratings my find their post lower on heirarchy)