From Diablo Wiki
"Fanboy" or "fan boy" is a term applied to loyal, often irrationally-devoted fans of some entertainment product or service (such as a game by Blizzard Entertainment). The term can be applied humorously, but is often used disparagingly, to mean a person who is blindly loyal and incapable of objectivity.
The passion of a fanboy (or fangirl) can turn abruptly, as their fandom burns so hot that a disappointed fanboy may turn very bitter and negative, perhaps even becoming a hater. This was a sadly-common phenomena over the months after Diablo 3's release, as many of the most rabid fans dealt with their disappointment by turning bitter, caustic, and hateful.