It wasn't what people expected. That's the main problem.
As for actual faults, there are many. Reused maps. Constantly spawning waves of enemies that make tactical positioning a complete waste of time. Armor and weapon customization for companions, or rather lack thereof. Ambiguous dialogue wheel - icons for brain-dead players, because who needs words when you can click at the fist and be done with it? The game completely ignores that you're a mage, you can use blood magic in front of templars and nothing happens. You don't relate to the protagonist - you're just an observer, you're supposed to feel things but you have no reason to (especially visible in the very beginning when your sibling dies).
Most of all, the story just falls apart - half of it feels aimless, the other half doesn't make a bit of sense and only the Qunari part is well executed. Even worse, your actions don't have any meaning. You're swept by the events and whatever you do, the end result will be the same. One could argue that mages vs. templars is an important choice, but even then the final scenes prove that everything both sides were accused of is entirely true. Again, the battle is pointless, that which you were trying to defend doesn't exist.
Also what they did to Anders is inexcusable. I never liked him, but he was a good character in Awakening. In DA2 he's... ugh. I don't even want to go there.