Oh man I've been waiting for a thread like this...
I believe that taken on its own merits as a game, Dragon Age II is by no means BAD. It's decently built fantasy Action/RPG. Between the way that the game fiction handles the nature of magic and the spirit world and the inclusion the unique race/culture of the Qunari, it has enough interesting twists on the "standard fantasy setting" to set itself apart from your average DnD-derivative shlock.
EDIT: Yes, I realize that this "unique" things were originally pioneered by Dragon Age: Origins.
I believe that the majority of the bad press this game gets is because it stands in the shadow of the superior Dragon Age: Origins.
I for was, and still am, a HUGE fan of Dragon Age: Origins. Yet, I still liked Dragon Age II. Why? Because in all the developer interviews, Bioware made it clear the new directions that they were taking, in terms of combat and storytelling. I clearly understood, before the game's release, that it was going to be very different from DA:O, and I accepted that. In terms of combat, I understood that the combat would be more hack-n-slash and action-oriented, and I downloaded the demo to get a taste of it first-hand before I bought it. In terms of storytelling, I understood that this was not going to be a sequel in the traditional sense, but rather a completely different story that was set in the same world.
These changes didn't bother me. I like slower-paced strategic games and I like fast-paced action games. To me, neither one is superior to the other. In terms of storytelling, I like that it wasn't your typical "save the world" story that has been done to death in video games, especially fantasy RPGs.
Did the game have its problems? Sure. The environments were recycled too much. Allowing the player to juggle so many quest lines at once caused the little "day in the life" stories to get jumbled together unless the player intentionally kept them straight.
For me, the biggest problem with the game is that they called it "Dragon Age II." Calling it "Dragon Age II" set up expectations of "direct sequel" in fans minds, and rightly so. Maybe if they'd called it "Dragon Age: Champion" or something similar that would have helped to highlight all the developer interviews where they clearly stated, "THIS GAME WILL BE DIFFERENT."
Then again this would still not be enough for some people. Some RPG fans look down on action games or action/RPGs as grossly inferior to more strategic RPGs. These types of people will be unable to see any of the merits of Dragon Age II because all they will see is "dumbed down." When really, it's not dumbed down, it's just a different type of game. If you don't like that type of game, that doesn't make it inherently bad.
In the end, I think that A LOT, if not the vast majority of, the fan criticism leveled toward Dragon Age II was because it was called "Dragon Age II" and that brought with it a truckload of expectations of what DA:O fans wanted the game to be like, what DA:O fans thought that the game SHOULD be like, and when it turned out very different, they turned on it.