Mother Russia
(Please keep in mind that this happened a few weeks ago and some of my information may be a bit hazy so I'll apologize ahead of time for any inaccuracies that may or may not appear :D)
Based on what I saw, you are going to play two humans: one female and one male. This was also present on the cover art and posters plastered all over :D I didn't get to play Original Sin and I didn't get a chance to really explore the main menu and such so I cannot tell as of right now whether you'll have the chance to customize the two main characters. I *think* one of the characters will be a magic user and other a fighter but at the time of the demonstration both characters had all the spells since this was to show off the various capabilities of the game and the engine.
IIRC, there are skills and I *think* you can also raise stats. AFAIK, some of the mechanics from DivDiv are going to be in the game i.e. the cross-class skills such as magic and ranger abilities. I'll have more on this later as I recall more (sorry again but like I said this happened a while back :D)
We didn't get to see too many magic items due to the short length of the demo.
About monsters: during the demo, the dev and another player fought some zombies at a lighthouse. That was about the only combat during the demo so not sure how common in terms of numbers or in terms of variety of creatures the monsters will be.
From what I gathered, the world is at war with the Orcs as evidenced by a quest where you get to decide whether to let a female Orc, who had a love spell used on her by a soldier, live or die (yes, there are consequences to this one too :D)
I didn't get to see any summoning parts.
If I recall anything else, I'll add to it :D
Edit:
About the DivDiv mechanics: I'll have to retract some of that statement for now due to the fact that I was playing Divinity 2, which featured the various skills and abilities I mentioned and thus my recollection got mixed up with that. So sorry, bros!
Rest assured that this will not be a popamole RPG.
Also, if you are playing with another person, when having a conversation with their character in-game, you won't see what they have to say UNTIL they make a choice in their dialogue. So yes, dialogue choices exist even between the two player characters :D.