Ghostcrawler, as you might expect, says it's a little bit of both. Blizzard doesn't just do what players say -- they consider player feedback and then make decisions from there. But at the same time, they can't ignore what players say, either. GC agrees that the forums are not the best sample of feedback, for the same reasons that Rekker gives: players go there because something is bothering them and they want it changed, not usually because they really love something in the game and want it to stay the same.
Of course, forums are not the only form of feedback from the community, and there are many places Blizzard can get feedback about things in the game that players like (ahem). But just like Blizzard does, whenever you look at the forums, you have to realize that you're looking at just a slice of the feedback. People don't make QQ posts about the stuff they appreciate and like having in the game.