I don't know how much pure hate actually surrounds DA2, there is certainly a lot of resentment and disappointment with DA2 though.
My biggest problem with DA2 isn't something that's really tangible (Though there were plenty of tangible issues with the game). Origins was hardly perfect, people who proclaim it one of the best RPG's ever or mark it as the best game in the past decade really are beyond my comprehension. It was a great, fun game but it had it's fair share of flaws and I
understand them looking and saying "we can do better than this". There was something Origins had though that I thought made it superior to it's competition, something that I've felt has always separated BioWare from the majority of the pack, something that felt missing from DA2. Origins had this incredible amount of detail put into every single aspect of the game and it gave the world, the story, the characters, the journey this incredible feel and you could feel the love and immense effort they put into that game. Almost everything looked and felt so fleshed out that it sucked you into the world and even though the focus was on your character it made you feel rather small and insignifcant.
For whatever reason, at least for myself, that feeling didn't translate over to DA2. It felt like they traded away substance and detail for flash and pazazz. The finely detailed world was replaced with rushed out sections crudley thrown together, epic and grand was replaced with quick and sexy. I'm not calling Origins flawless nor am I calling DA2 bad, but there is a stark difference between the feeling of two. Perhaps it's solely because Origins had a couple more years and several million more in spare change, perhaps the culture and leadership allowed for more during the development of Origins than in DA2. Perhaps it was lack of time and resources, perhaps it was lack of will, or perhaps it was a little of everything.
And again there are plenty of tangiable reasons for me to have issues with DA2, but nothing that felt like it was hurting the experience more than the actual experience. And I had my problems with Origins as well, the story and characters were rather trite, the flow of time and urgency was lacking, clunky combat, and other little things here and there. But to me Origins was like the movie/miniseries Lonesome Dove, the characters and the story were nothing overly special and at times it moved slow as hell, but there was just something to it that sucked you in, something crafted so finely that it felt so much more grand than it really was and made you feel more for the story and characters than you should have. DA2 was more like a Micheal Bay flick, something that leaned more towards style rather than substance, where instant gratification took precedence over detail and thought. Chances are you came out of it satisfied and enjoyed it, but it's not the same feeling you'd have after seeing The Godfather or Shawshank, and it's fair to leave wanting more then what you got. When you set the bar in the sky, falling short will create disappointment and even resentment.