Okay, my turn. :}
What makes a fanboy/fanboi?
Lack of objectivity.
It's pretty much as simple as that. Anyone willing to overlook (and/or ignore) the facts of a situation is pretty much lost in the first place...lol. But, there are many instances where people don't understand the difference between a fact and a preference.
It's like asking if Mortal Kombat or Street Fighter is better...you will get people that side with either game for (usually) personal reasons based on preference. While there may be some actual "facts" that could be pointed at (graphics, control issues, etc.) for any game, the fanboy types are usually (okay 99.9% of the time) unwilling/unable to admit there is ANYTHING that can be seen as negative about their favorite.
ANY game is (and should) be considered fair game when it comes to evaluating/reviewing it for pros (positives) AND cons (negatives). As cool/neat/pretty/fun any game can be, that does not mean there is absolutely no room for improvement. Some people think each game should only be reviewed/evalutated on it's own merits and not compared to what else is out. That's a nice idea, but it just doesn't work all that well (mainly because we live in a comparative type world).
However, that does not mean a game can't stand on it's own. But, other games on the market must also be taken into consideration when reviewing/evaluating something. [However, that does not mean reviews (especially in magazines and/or game websites) should focus more on the game being compared that the game being reviewed...yes, I've seen that too.] In any case, each and every aspect that makes up a game should be evaluated fairly (both the good and bad points about it).
I think part of what hurts the ability for people to be objective about a game is some of the sheep mentality that allows them to accept a game based on it's "popularity" instead of the actual game itself. Hype can go a LONG way into helping propel a game into a higher status area when it truely isn't as great as all that...lol. For example, how many people rushed out to buy Halo 2 that had not even played Halo 1? (more than you would probably imagine). It's just too easy for some people to get caught up into something that's popular like that.