Well, here goes, why not.
1. The Plot. There is none. The game is about Hawke's rise to power, but that's not a plot. It's a character arc. The hero's rise is an intrinsic element of any classical fantasy tale. However, his/her rise must always be driven by some main, overriding plot. Taking that arc out and trying to make the story just about that might work in some story types, but not in a linear, structured narrative.
2. Combat. There were actually a couple things wrong with the combat, so I'm breaking it down into subgroups.
a. Mechanics. The combat this time around was designed to be faster, more impressive and more engaging to the player. For many people it failed all but the faster part. Stating why it failed is a matter for opinion I suppose, but I think it was because it was trying to be two combat systems at once. Part of the game was trying to be a brawler, with button presses required for each attack/skill use. The other part of the game was trying to be more like Origins: tactical party-based combat. Trying to be both of these things, I feel, made the combat lacking all around. It didn't have the control and finesse that good brawlers do, but it couldn't be a good tactical game because of many of the mechanics (speed, lack of to-hit, etc.).
A parallel I like to draw is the difference between the Witcher 1 and the Witcher 2. The two games had radically different combat systems, but fans didn't begrudge the switch because the developers went the whole way and made it a damn good brawler. Furthermore, combat in the Witcher 2 still felt like the Witcher 1, even though it had a radically different system, because of how it was presented. Which brings me to the next topic:
b. Presentation. Combat is disturbingly poorly presented in DA2. There is hardly any build up to most of the fights, for one thing. If you do anything but walk around the city in the daytime you will be attacked, but the battles are over as fast as they began and no weight or consequences are ever attached. This makes them feel incredibly artificial, like they're happening in another dimension or something.
Enemies also have no impact since most of them are nameless, faceless, nobodies that are jsut there to make Hawke look awesome. There are also too many of them. A tough fight in Origins might have a single mage backed by four warriors, but DA2 never presents a challenge unless there's a boss and a dozen cannon fodders. This goes further to making to the fights seem artificial.
The last thing the game fumbles with the presentation is the animations. They're incredibly visceral, fast-paced, and cool-looking... for the first couple of hours. Then they just start feeling overdone and boring. There's no sense of weight to the character's movements, and the way they swing their weapons makes it look like they're using blow-ups rather than real weapons. It's the final nail in the coffin for making the fights seem fake.
On the other hand, the magic effects and models look much nicer than they did in origins.
3. Characters. the final thing I'll talk about are the characters. One of the big changes this time was the main character had both a set name and a voice. Yet, somehow they managed to have less personality than the characters I played in Origins. I think a large part of that is tied to the fact that there is less dialogue this time. Since the voice actors had to record 3 responses to everything someone said, the writers seemed to compensate by giving fewer chances to respond and shorter lines.
Another reason might be that all responses this time around (or nearly all) had to be shoe-horned into the Nice, Sarcastic, and Asshole categories. This meant there could be no more than three responses to anything, and they all had to fit their predetermined roles. Origins, on the other hand, could have a multitude of responses, and they left it up to the player to decide whether the character added a sarcastic tilt to it.
Which brings me to the voice actors themselves. They both suck. Moving on. No seriously, the voice actors for Hawke (both genders) are easily the worst of the main cast. Why? Why do that?
Anyway, the only other thing I'll say about the characters is that it was very hard for me to care about my companions since most of them had no logical reason to follow me around. Especially if they didn't like me. In fact, my first meeting with Anders went very badly since I thought he was an ass for leaving the wardens. Yet, despite my snide remarks and his open hostility he was more than willing to follow me around and obey orders. I also never took Aveline since THERE IS NO REASON FOR THE CAPTAIN OF THE GUARD TO BE FOLLOWING ME AROUND TOWN SHE HAS A VERY DEMANDING JOB SHE DOESN'T HAVE THE TIME!!!!
Conclusion
Despite all that I did enjoy DA2. It's a decent game, if nothing else. It just makes a number of mistakes that I largely think is the result of not enough development time. It would probably have seemed better if it wasn't a sequel to a much better game. I'll be interested to see if BioWare can turn things around with the third installment, or if they'll just do a rush job to milk whatever is left of name recognition with the series.