DA2 is different from DA:O in that elements of Mass Effect are incorporated into it. Also, it is true that it rehashes a lot of the same areas/maps. I think this is why people think it's a rush job. I disagree with it being a rush job and believe, instead, that this was intentional.
My biggest gripe isn't that it's shorter than DA:O or that the new combat style and character customization has changed. Rather, my biggest gripe is that I got so sick of looking at the same city and the same dungeons with the just entrance/exits moved, different part blocked off, and no reflection of that on the overhead map/nav guide. I rushed to beat the game, simply because the same scenery was creating the feeling that I could not break out of this place, that I was simply trapped!
Anyways, the combat system has changed to a Mass Effect style. My play-through was as a Mage. Despite the magic casting options, Mages will still use their staff to attack quite a bit. Think of a Mass Effect Biotic armed with a pistol, of course the pistol is a lethal weapon and they will use it, it's just not their bread & butter. Same with Mages, staves aren't what you use only when your mana runs out, you'll be using your staff much more often than traditional RPGs. The changes in the combat style create more excitement within the game. Everything is fast; although you can still pause the game at the press of a button to change up your strategy or think about your next move, pausing isn't necessary on the normal/medium difficulty for most battles.
Also, the character customization has been dumbed down. I don't know if it's been improved or if it's worse, but it's definitely simplified. Instead of being able to have access to the different lines of spell schools/types, you have spell circles and trees. You may not even fill out a whole circle, instead choosing only the upper or lower half. The same for trees, you might not even want to fill out the whole branch, just selecting enough to grab crushing prison.
The same is done for fighters and rogues, by the way. Circles and trees, not lines, so you need to do some short-term planning for what you want, but you will not need a guide just to figure out what might be decent. I believe this was changed to go from a hardcore RPG audience to a more casual RPG audience...
Lastly, on customization, the character you're looking at will have a particular set of talent trees to look at. You don't get to pick the Templar or Beserker specialization for whichever fighter you like. The specific character will already determine if one of those talent trees are available to you, based upon the developers' determined fighting style of that character. This helps create variety and potential use of all the characters introduced in the game, but it definitely removes more of the customization from it.
These are the significant things I've noticed about the game, compared to DA:O. Despite my frustration with being stuck in the one city for most of the game, looking at the same dungeons, and the overhead map not updating to show me which sections of the rehashed dungeon are blocked off, I did enjoy the game. The new changes were fun, even though they are a large step away from traditional RPG systems and even a large step away from its predecessor (in name and story only), I think it gives gamers something they may enjoy on a more casual level. However, I'm questioning how much overlap we'll see, in the future, between Mass Effect and Dragon Age and if these games will simply become the same game as the other with differences in story and background, alone.