Picking up on yesterday's social media cocktail party theme, when it comes to moderation, I like the "please use a coaster approach". This means that all opinions are welcome as long as they follow a small set of house rules. Things like: no anonymous comments, no spam, and no cussin' -- either the "your product is shit" or "your product is the shit" kind. Basically, everyone's more than welcome to agree or disagree, just be respectful.
This is where the coaster analogy comes in -- if I throw a party at my house, I want all of my guests to have a good time, but I don't want rings on my coffee table when they leave for the night :-)
(The coasters pictured above are available at socialdrinker's Etsy store.)