First, I'll start off by saying that DA2 isn't a bad game - it just has some flaws, and isn't nearly as good as DA:O.
The most striking flaw - which was no doubt a result of the fact that DA2 was produced on a far too tight schedule - was the recycling of areas maps. All dungeons and houses, and other such areas in the game use and reuse the same few layouts, that are simply recylced. So a smuggler cave at one location, may have the exact same layout as a, say, slaver cave at another location.
This, added to the fact that the whole game happens within one city, and a few locations in it's immediate vicinity, gives you the feeling that you're running around in circles, revisiting the same places again and again.
Another complaint is the storyline - it is a very unconventional story, in that you really don't seem to have much of a "grand purpose" at any point, but rather, in trying to get ahead, stumble upon momentous events, and only after the fact you can see what the game was building towards. Many people didn't like this, and I do think there's some truth to a comment I read a short while ago: that all the quests feel like sidequests, with no main storyline.
The storyline is definitely hard to see, on the first playtrough.
There were changes that not everyone liked - like the fact that in DA2, you cannot customize the armor of your companions; they have their own specific armor througout the game that you cannot change, though you can find upgrades to their armors.
Also, you play as a set character - you can select his/her gender, class and appearance, but not race; you play a human named Hawke, and there are no different origin stories, like in DA:O.
There are good points to the game too though - like character writing. You meet some interesting companions, and the banter between some of them is great. (Merrill, I think, has the best, often laugh out loud banter, with pretty much all the other companions)
What some people liked and some didn't was the combat - combat is faster paced in DA2, and the moves faster and more exaggerated. Enemies exploding to critical dagger kill blows, and stuff like that.
What was controversial was the combat waves - in combat, enemies come in waves, and new waves of enemies seem to drop to the battle field out of thin air. What is better though in DA2 combat (and it may be due to the waves enabling large battles without having too many enemies on the battle field at any given time), is that there is no lag even in big battles. What annoys me most about DA:O is that at least on my machine, I sometimes get terrible lag in the large battles with many enemies on the battle field.
I do suggest you play through it though, after DA:O. Just don't go into it with too high expectations, and you may enjoy the experience. And DA2's storyline, without doubt, sets up the setting for DA3.