Here's a hint concerning Polygon: Arthur Gies. (Yes that is a joke article but having someone with such a record as your review editor isn't good for the credibility of your website). Also they don't seem to have a quite so big community if you watch the latest replies on the recent threads: Not a wasteland, not buzzing either.
Yeah, I was sort of surprised to see that some of the sites that have a lot more traffic than Giant Bomb, Polygon and Gamespot, seem to lag behind in terms of community interaction. The forums over on Polygon are dead and they really aren't all that active over on Gamespot either.
System Wars is where all of Gamespot's forum activity is. Also, every time they post a news story with any sort of console war slant (be it sales numbers, resolution bullshit, or anything about an exclusive game) there's pretty much guaranteed to be a minimum of 1500 comments on the article, the majority of which is complete and utter trash. There is certainly a lot of individuals contributing on that site, it's just that most of the discourse is awful.