MrHaga, you are right on! I'm pretty sure that's a major component of it. Also, the people who post positively tend to get criticized heavily for being "Blizzard fanbois", which is an incorrect spelling of a pseudo-word. Like pwned, I hate seeing that. Owned is almost acceptable, but pwned (to me) screams "IDIOT!"
Also, some classes were far more powerful once than they are now, and complain because they resent the changes since then.
Exhibit A: MAGE. The patch after mages got talents was horrifically imbalanced in favor of mages. They were dominating everyone, and the mage population exploded as people tried to get in while the getting was good. (Side note: They defended their overpowered class by saying such things as, "Don't make us weaker, make everyone else stronger!" and "Don't ruin the ONLY fun class in the game!") Mages have been heavily nerfed since then to bring them better in line with everyone else. Result? There's still a large number of mages, but many are bitter over losing access to god-mode.
And so it goes. Many classes had times where it was overpowered or had things only it could do. When these are changed for balance purposes, people complain bitterly and harbor that resentment with them over time.