How is DA2 combat fundamentally different from DA:O? That seems to be a popular question in these parts. From a personal perspective there's a clear difference and I'll describe why I don't like it.
I used the overhead perspective (isocam) almost exclusively during combat and 60-70% of the time in exploration mode. I would use a stealthed rogue to lead while the rest of my party would trail. The rogue would disarm/set traps and detect ambushes. Since I play on higher difficulty settings my preferred method is to minimize exposure to getting blindsided and/or taking damage that can be avoided. I like to be slow, methodical and keep situational awareness at all times.
Let us suppose that my rogue detects a line of traps at a choke point around a bend in the dungeon. Not only that, but there are a dozen derpspawn and an emissary waiting in ambush. Using the isocam I can have my stealthed rogue disarm the traps and set some of my own traps in their place, while I can keep an eye on the mooks to make sure they're unaware of what I'm doing. Then my rogue can launch a scattershot while the rest of my party moves into a forward position. My mage can accurately lay an AoE on the enemy while they're stunned because I can detach that isocam from the character and aim it precisely where it needs to go. Meanwhile I put everyone else on hold to keep them out of the danger zone until the effects of the AoE wear off, all the while my ranged fighters are getting in some free shots. Once it's safe I can send my tank into action and have my ranged party members lay down some devastating crossfire. I'm controlling the action from a roaming, top down, battlefield view and pausing when appropriate to issue commands. The result is that I thwarted an ambush, turned the tables on the enemy and minimized the damage to my party. I didn't have to keep a mage dedicated to spamming heals and revives. My crew don't have to quaff health pots every few seconds (although health pots have a cooldown now so...smh) to be effective.
Now obviously not all battles can be so easily manipulated and on open ground the combat can get chaotic at times. The fundamental difference is that by using the isocam, I can pause and target AoE attacks much more precisely that I can with the restricted view available in the DA2 demo that I played. Playing on nightmare difficulty it's going to be damn near impossible to keep from nuking your team accidentally in the heat of battle. I've read that the EAWare dev that completed the game on nightmare essentially had to rely on the lightning/electrical tree of AoE spells that's exempt from doing FF damage because of this. Let's not forget that now the melee fighters do AoE FF damage as well, so you better keep that in mind.
One other change is that they seem to have nixed spell combos and instead are relying on cross class combo action. That sounds like a nice challenge in regards to timing attacks from different characters, but how does that work unless you turn off 'tactics' and control all abilities manually. You can set up a combo only to discover that the corresponding team member ability is on cooldown and the opportunity gets wasted.
These are some essential changes to combat that preclude me from playing the game the way I did in DA:O and consequently the way I prefer. There are other things I personally dislike about the design of this game, but the dumbing down of the combat mechanics are my main gripe.
tl;dr - No isocam, no sale.
