Well for me personally it wasn't that it was bad in comparison to DA:O, just thought it was bad because it implemented things it was trying to do poorly.
The one city setting: The city never changed in ten years, it was empty dead and lifeless. If you're only going to have one city it should be bustling, you should be able to interact with it, invest in it. You couldn't do it even after you obtained wealth.
Family: The game never gave you a reason to care about them. They were only there to kill off in an attempt to make it dark fantasy. You had all these cutscenes with your companions and the dog. But how many times did you come home and talk to your family. You were in the city of your birth IIRC and how many quests did you do pertaining to your family in this "personal" story.
Personal Story: You were told way more than you actually were allowed to play through. If the first act was going to be random quests, why not let you plat through the part where you made a name for yourself. Do random quests as a mercenary or a smuggler. Maybe that's where you meet Varric, or someone close to Varric and that's why he pick you out to help him with the expedition. That's just one example of where the game could have allowed you to play through more of your story.
The Ending: Just preposterous. Orsino talks about having hope but feeling that with the Champion there they may just have a chance. You beat back the Templars and then inexplicably he chooses that moment to go crazy and not only use blood magic, but attack his allies, the survivors from the group he was just sad about those who had died.
Then I don't even think I need to elaborate on the wave combat, enemies falling from ceilings and the sky, or the horribly reused environments.
This is why I never understand the whole people didn't like it because it wasn't Origins. All games have flaws, the questions is the ratio of things it did well to things it didn't, and for me there were far more things that it did wrong than right.