There are many factors at play ...
PS3 fans are generally the most insulting towards users, games and systems which is why they're banned a lot. I think this is somewhat natural as current PS3 fans are probably both fans of Sony and believers that 'Graphics are the only way to advance videogames' and are probably frustrated by the success of Nintnedo systems.
XBox 360 fans are generally quiet because the XBox 360 is in a very "netural" position on most message boards; they share the argument that "Graphics Matter" with Sony fans yet can share the argument that "The PS3 is too expensive" with Nintendo fans. Beyond that the XBox 360 has taken a lot of beatings to this point in time and even the fans are probably willing to admit that it has its warts.
The Nintendo fans are somewhat smug, and often overly smug when responding to the negative comments from PS3 fans. A large portion of this is that they are the people who believed in the Nintendo DS and Wii from the beginning and were constantly defending Nintendo's approach; the fact that they were right on a level that (probably) surpassed their expectations has made them this way.
