Gameplay wise:
-Better camera. If they could bring back the DAO camera for DA3 id be really happy.
-Less enemies, stronger enemies. The 2 waves of random spawning "5 critter 3 normal 1 elite" or whatever enemies of DA2 was just plain bad. It takes away a lot of the tactical combat when a whole wave spawns on your healer out of thin air for no reason. It's okay if there are fight with a lot of enemies, but there should be a trick to these fights, like terrain advantage, not the "just overpower everything while spamming threat control abilities". Legacy did it better, but it was kind of sloppy imo. Also the first fight with the crows in "mark of the assassin" was pretty nice.
-This wasnt good for dao as well, but I'd like to be able to attack while moving. It's pretty important for backstabbers and archers.
-Cross class combos, oh boy, that was hit or miss depending on the class you played. Playing as a warrior that can stagger all the time, it's great. Playing as rogue, it's annoying, I had to use varric as obscuring bot. Playing as mage... meh, better if you use tactics and play other characters. I really liked the dao approach. Hex+cloud, storm of the century, frost+crit, forcefield+prison fun times.
Story/plot
- Dont care about races available for the hero, but I really enjoyed the prologue stories in dao, provided that I only played the human ones

. Sets the mood right for the tragedy later. In da2 it was a tragedy within a tragedy within a tragedy and we only saw the boring tragedy.
- The companions. Morrigan was the introvert manipulative **** that you know absolutely nothing about other than she loves you if approval is high, Leliana was the goody2shoes with a history of extreme violence that takes a while to kick in. But the best thing is the way those characters evolve and the conversation you can have with them. In da2, you dont have conversations, everyone tells you outright about their mission, none changes/evolves except Isabela in the end of chapter 2. Aveline is aveline, anders is anders. even goddamn fenris is demystified in the very first conversation you have with him. There are only two instances where you feel like you influence those around you like a leader would, and this is when isabela returns with the tome in the end of act 2 and fenris joining you against the templars in the end of act 3. I enjoyed Hawke way more than the Warden but the Warden felt like a leader while Hawke just some overachiever.
- The Nemesis (that isnt what he seems to be) and the great cause. Bioware does that pretty well. Darth Malak is some Sith that likes to kill people, till you learn that you are Revan himself and Malak is your former best friend. Saren is an intoctrinated agent of the Reapers that want to destroy the world but his intent was to persuade them that the world was worth saving. Loghain betrayed his King but he wanted to protect Ferelden against it's enemies while the burden of what he did weights heavily on him. Hawke doesn't have a nemesis, he is a son to a mother and a brother to a sister (or brother), and he tries to make the ends meet, but he is skillful and is presented with a lot of opportunities. It's difficult to understand what makes this guy that important.
Can't think of anything else right now ;p