Overhype (which doesn't mean a game is bad, but that people set their standards unrealistically high).
Also, people love being "that guy" to nitpick a critically acclaimed game/movie/restaurant/food/celebrity/anything that can possibly be acclaimed. What you end up with is a lot of "those guys" posting on forums (because really, "those guys" thrive on letting people know how hip they are for disagreeing with the mainstream).
Also, those who have negative experiences (with ANYTHING) tend to be much more vocal than those who have positive or as expected experiences. No one is going to hop on a message board and say "Guys! My toaster works as expected." But if some people have a broken toaster, oh my God, forumgeddon.
Other possible explanations:
- Not being the game they wanted ND to make, so they get mad and disappointed
- Not controlling how they wanted it to control, even if the controls are fine, so they get mad and disappointed.
- Taking one instance of a bad experience (an AI stupidly running into the open, for example) and extrapolating that out as "the AI is bad."
- People expecting the game to be Call of Duty Zombies mode.
And of course, there are some people who are being 100% honest and genuine about not liking the game.