How long has this site been established?
It would be interesting to see how the forum posters' bias has shifted from last generation to this, because personally I think it just has to do with the numbers.
I bet last generation Sony fans were doing the same thing that Nintendo fans are currently doing. I bet if Xbox had the upper hand next generation the bias would shift in its favor. Simple as that. Whoever wins will have the majority of voices, so this bias is perfectly understandable and is expected.
Of course, if you want to get rid of the "fanboy wars", setting up something along these lines might work:
Nintendo Forum
Sony Forum
Microsoft Forum
Warzone Forum
...pretty self-explanatory stuff. Trolls, flaming and all the "negative" vibes are concentrated in the Warzone Forum, and if you post there, you are fair game.
The CVG forum is pretty much like that (sans Warzone Forum) and you can see it's quite civil.
Anyhow, my two cents'...
