MaKe dEnny uSe proPer Capitalization.
Prohibit all flame wars and personal attacks and enforce it hard. It drives away a lot of really good people who aren't into being personally attacked. There is no reason for members to treat one another disrespectfully. Even a dive bar makes you take it outside.
No politics or religion. You want to call Donald Trump a c***sucker, no matter how justified, go to Facebook.