One of the more common gamers that are full of preconceived notions are fanboys. If you're too stubborn to realize you are a fanboy, just reference Jim Sterling's Ten golden rules of videogame fanboyism. If you can identify with some of those rules, you are most likely a fanboy. In fact, if Jim burns you up with his "editorials", odds are, you're a fanboy.
Now, there is absolutely nothing wrong with being an avid fan of a specific console, publisher, developer, game, etc. However, when one is unable to handle valid criticism on the topic, continually creates excuses, or has nothing but complete disregard, this is a problem.