I can only give initial impressions since I'm just finishing Act 1, but like other posters, I have a love/hate reaction toward BD so far.
The combat seems a lot clumsier and much tougher, controlling two characters in what is essentially an action RPG ruins the flow of combat...attack, pause, potion, unpause, hit, hit, cast spell, pause, potion, unpause, attack, attack, RUN, death knight dies, GAME OVER press escape. I hope Larian doesn't incorporate this into DD2, but I wouldn't mind if they had NPC's that joined you, only computer controlled to some extent, and if they die it shouldn't be game over.
I'm a little underwhelmed by the skill system also, it's not so much complicated as convoluted. Each skill tree unfolds like a road map and there seem to be miniscule distinctions between a lot of them. I just want a fire spell, not have to decide if it's "focussed" or "instant". And if my fighter chooses "One handed, one eyed, two fisted, no shielded, slashing with a double edged dagger" skill and then finds a better weapon later that doesn't fall into this category, sure I can unlearn the skill, but that costs money, and soon we have no money, and neither do the merchants anymore. We're all poor!
Battlefields are an interesting concept, and the game is designed so you pretty much have to go there if you want to progress in the main game. But this travelling to an alternate universe kind of takes away from the immersion factor. Am I going on some kind of killing/questing vacation? How about I just set up house in the battlefield and just forget about getting out of Samuel's dungeon? I kind of like that idea..
Attributes are weird, Strength is clearly not as important as Agility, even for a burly fighter. Intelligence gives the mage more mana, but never enough to make their low level spells very effective. I'm not sure where to put level up points anymore.
I truly appreciate Larian's attempts to make BD a little different, but there is a point you cross into the "fixing what isn't broken" area.
I have broken this post up into paragraphs to facilitate quoting and flaming, you're welcome!
