Many have noted upon the key issues already, but I would say the main issue with DA2 is that it was a disappointment. Still a solid and enjoyable game, but rather lacklustre compared to Bioware's other games (even just the recent ones). It made quite a few experimental changes as well, which led to some criticism...
... which eventually spawned the "Bash DA2!" bandwagon.
That fiasco can only be described as a mob mentality, which isn't exactly known for bright ideas. Unfortunately, it would seem that some of the effects still hold sway over many; the focus on the negative aspects of the game prevents them from seeing that the game still has some good points.
As for some of the reasons why DA2 got a less-than-favourable reception:
Excessively recycled environments
Reusing actual locations after time has lapsed is one thing, but there's no excuse for how much it happened in DA2.
Overuse of wave mechanics in encounter design
The combat in DA2 is better than DA:O, the encounter design however? Well, the unique fights are better than anything in DA:O (the Ancient Rock Wraith, for example); but the general encounters are rife with problems. Additional waves of enemies can be used well, but DA2's implementation is done poorly and done far too often.
The characters are less likable
Now, this isn't to say they're worse; just less likable. Or if you prefer, less idealized & archetypical. One of the potential issues of creating characters with more depth is that people will start to dislike them due to their flaws rather than sympathize with them. An emotional equivalent of the "Uncanny Valley", or something along those lines.
The story's execution is a complete mess
The three acts of the game are very disconnected, dealing with three very different conflicts. The first act in particular is practically aimless, and the other two just don't flow well. While many of the over-arcing concepts and smaller events are done well, the whole of the game's story needed to be planned out better. Also, the game seldom demonstrates that time has tangibly passed between each act of the game; that was a lot of wasted potential right there.
The lack of interaction with party members
One of the nice things about DA:O and both games in the ME series (I haven't played any older Bioware games to know if it's a persistent trend) is that you can talk to your party members at any time; or at least any time you're in the "hub" location, be it the "Camp" in DA:O and the Normandy in ME1/2. In DA2, you're restricted to talking to your party members at only specific points... yeah, there goes a lot of the interesting dialogue trees right there.
Players feel they don't have a strong effect on the story
Now, is this really a bad thing? To tell the truth, it isn't... but don't be surprised with many people don't respond well to feeling either vulnerable or ineffective. Removing the "escapist" aspect of a medium where just about every protagonist is an "escapist character" (read: borderline Mary Sue) tends to a very risky move. The same can be said for other media (films, books, and so on), but it's an especially dangerous prospect for games; players want to be nigh-omnipotent, games are treated as a power fantasy; DA2 subverts this by having a plot where there is no possibility for a comprimise, due to all sides of the conflict being both right... and wrong.
People hopped on the bandwagon
Yes, it's worth re-iterating.
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Now, DA2 does several things right. The combat is probably the biggest improvement, with more balanced classes and the addition of cross-class combos. The game also runs a LOT better than DA:O, even though the combat is much faster paced. Gifts now how dialogue specific to them. The story of the game is far from the generic tale that DA:O was (though the earlier game did have a much better execution of that story), and DA2 does a good job of showing the moral ambiguity of the setting. The game also isn't the sort of game many were looking for (DA:O 2), which probably didn't go over well when combined with the definite flaws of the game.
The main issue, again, is the poor execution. If DA2 was planned out better and had more time to refine itself, the only reasonable argument would be that the game wasn't the type of game they were looking for.
