DA2 certainly could've used some more development time to iron out a lot of the issues; so here's a (brief) commentary on what should've been done, starting with the obvious ones:
1. More variation in environments
The excessively recycled environments of DA2 is inexcusable, no matter how you slice it. ME1 did this a bit as well, but it was at least restricted to the sidequests. With DA2, it's more persistent throughout. It's an obvious sign that the game didn't get sufficient development time as well.
2. Properly implement the wave mechanics for encounter design
The issues here are two-fold; waves were over-used, and enemies appeared out of thin air. I feel this is most prevalent in the first act, as it seemed to be dialed down afterwards; almost as if the first act was the last one to actually be made. Used (far) more sparringly, wave mechanics are a good idea; a way of keeping players on their toes, by forcing them to think quickly & act decisively. However, when it happens in every single encounter? The player comes to expect it, and it actually becomes annoying. Enemies should also not just teleport in, there should be some mechanism to their arrival; fighters run around corners, mages teleport in (rather than just appear out of nowhere, there's some magic effects shown; perhaps even a portal), skeletons come up from the ground, and so on. Wave mechanics aren't an inherently bad idea, but the poor implementation and excessive use in DA2 is a problem.
3. A better paced & planned first act
As noted in my previous point, the first act seems to be made last; and made hastily at that. The Deep Roads Expedition is one of the best parts in the game, but hours leading up to it is practically aimless. Gathering your team isn't too bad, but there's very little connecting them to the main storyline (at least at that point). It also drags on, especially for completionists who want to do everything. It just feels like a bunch of random adventures, and quite a bit of it just doesn't lead up to other details.
4. A more coherent central storyline
The mages versus templars storyline should've been given a lot more focus in the first two acts, and both Orsino & Meredith should've shown up (and had a greater effect on the plot) before their appearance at the end of second act.
5. You should've been able to talk to your party members at any time
Dialogue trees being confined to only specific conversations makes them feel a lot more shallow than DA:O's cast (though the depth DA2's cast is at least comparable). Hawke should've been able to sit down and talk with them, even if just to talk about stuff not relevant to the plot itself.
6. Progression of time should've been shown more
Very little changes in Kirkwall between the acts, especially in the visuals; the time-skips don't feel like much as a result. Rather than barely any notable changes, more should've been done; plain & simple.
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That's my take on DA2's problems at least. It's a great game, with several glaring issues; if they were taken care of (and they're the sort of thing which should be taken care of), the game would have been much better.
Of course, that doesn't solve the issue of players wanting easier-to-understand characters & plots (DA2 is far more grey than DA:O)... but not much you can do about that without comprimising the story DA2 was trying to tell.