The short answer would be a biased person towards his video game system of choice only.

But when I think about it, in the first place, I only bought a PS3 because it is my gaming system of choice, since I prefer Sony products, I trust the Playstation brand, and I want to play Gran Turismo. That makes me biased, since I'm preferring one system over another based on my own set of criteria. But that doesn't make me a fanboy, does it?

Aren't we all this way? That, in some form or another, we are biased due to some personal criteria? In this way, it can be determined that a fanboy isn't simply biased.

So what is it exactly that makes someone a gaming fanboy? I'm trying to point out a singular mannerism that is common in all fanboys that we know of. I mean, we know what the fanboys say all the time, but do all of them have something that 100% of them do say? 

Deep within my heart I know GT5 will be better than Forza 3, but I will force myself to like Forza 3 more because I'm an Xbox fanboy.

So you're saying a reversed bias attitude? That even if a person knows that something is better, he'll still play the competitor's game just for spite?

Fanboys may become so fascinated with the objects of their infatuation that they become obsessive. These fans engage in behaviors that are considered extreme or abnormal.

Exactly. That's the core nature of fanboyism. At least for me.

Always praise your own boys and downplay the enemy.

What I think -

1. Freely advertising for a company that doesn't pay you.
2. If you say "(insert company name) fanboys are the worst," it is probably because you are a fanboy as well of a different company.
3. A compliment toward a non-preferred company is taken as a threat toward your company.
4. Judging a game based on the console it is on and not the game itself.
5. Following your preferred company's decisions whether you would originally agree or not. The Rush Limbaugh "Ditto" syndrome, as I call it.

If you study marketing, they are "loyalists with no competitor substitute."  They advertise your company without having to pay for them.

like "SlimeBeast" Always praise your own boys and downplay the competition (enemy?!!).

^ I meant 'the competition' yes.

Basically. the people that refuse to admit that the competition has any strengths at all or does anything better than their own console.

                                                      Suspicious