HFB1217 The single most influential factor for a successful and respected discussion board is strong moderation. The fact that it could be a "closed" forum (registration required) does not change the fact that moderation is still required.
Political (and religious) posts will stress moderators to the extreme. I feel that politics has no place in a technical discussion forum. I only joined one of the political forums in DSL Reports was because it was offered.
You do need a few of these areas, as you need to not only attract new traffic, but retain that traffic. The "off-topic" subjects do that. Hobbies and special-interests require a minimal amount of moderation. In DSL Reports, I was a frequent poster in the photography and Team Helix sub-forums.
