1) Darker & more realistic graphics and grimmier atmosphere

From comments in the blog post Divinity – Original Sin revealed !

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Well, the truth of it is that we actually haven't spent a lot of time looking at the lighting. There's still some layers of graphical detail that'll be added to it, and I think as development progresses, you'll see a lot of improvements.
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I think you'll see a difference when E3 footage comes out already - we didn't really decide on the lighting yet & are fooling around with it for the moment.


2) Being able to switch between real time and turn based combat

Those are fundamentally different designs...


3) At least 4 class trees to chose from [first Divinity lacked a bit in this department]:

Four of the six skill trees are magic (Earth, Wind, Fire and Water). Both characters can be developed as you wish (thieving skills have not been mentioned, but I'd guess will be available; warrior/paladin skills are pretty much guaranteed).


4) Kiril Pokrovsky's score

Of course.


5) Randomized loot system

Also a given.