The short version:
Divinity: Original Sin aims to be a modern day Ultima 7 and is reminiscent of many of the older games in the genre, it will make many of you who did like games such as Baldurs Gate II and Planescape Torment very happy indeed, especially if you also like turn based tactical combat. Players will be able to create a team, make choices and see how the consequences of their actions bear out.
The long version:
This is a game with little to no hand holding and it does not insult your intelligence by making this game accessible to your five year old sister, no this game is a deep tactical turn-based RPG experience. Not only will you have freedom over your choices in the story (consequences and choices, even disagreements, will affect how things go), but also in how you get there.
You will be forced to interact with the world and speak to the people in it to gain information and quests, this is especially true as there are no exclamation marks over their heads telling you that this person is important or that they have information you need. It's also a game that rewards exploration, should you be careless enough you could find yourself completely missing out on certain quests. You can even kill most npc's in the game should you wish to do so (with the exception of a very few plot specific ones), but be careful of what you decide to do, there will be consequences for your actions.
There will be puzzles, lots of traps and mysteries to solve and interesting ways to approach many of the quests, you have the freedom to go wherever you want and the areas/monsters -wont- scale to your level, meaning you could die a very horrible death by exploring too far when you are low level. You could also be very skillful with the items/weapons at hand and somehow actually kill that level four troll while you are just a meager level two knight.
Being a rogue is especially fun as you can steal nearly everything, including snatching paintings off the walls, pick pocketing their weapons or even keys to their homes for a later "visit". Stealth is also very viable when stealing, but be careful, if you are caught people will dislike you and that will affect prices and information you can get, eventually you will be thrown into jail. Once there your partner can either come and bust you out or... well.. i wont spoil the surprise for you, suffice to say that there are more ways than one to get out of jail.
Party-based turn-based combat will encourage players to shape and control the game's environments, and to discover and utilize various skill and spell combos that you can choose for your party members
But the best part is the superb co-op experience, gone are the days when only player 1 gets to decide upon important decisions. You can both talk to any and all npcs and you both take part in dialogues during important events or with certain npcs, and should you two disagree on how to resolve a quest or dialogue you can have your characters argue over it. They will also react to certain things, for example, if you steal something your partner can start up a dialogue with you to either condone your actions or condemn them.
As to not spoil anything in the game this example is merely fiction and not present in the game. (Not to my knowledge at least)
example: You find a valuable gem on the beach and player 1 wants to sell it while player 2 wants to try and find the owner of it, stating that its immoral to take such a valuable object from someone who obviously dropped it.
Player one then gets a few different dialogue options to try and talk some sense into the other player's character, he can choose between a string of diffrent types of retorts and some might be intimidate, persuade, charm etc. After he has chosen his retort player two can choose a counter argument and will utilize the skill you chose and a game of rock, paper, scissors will commence, you will utilize the skillpoints in that particular skill you choose to gain an advantage over your friend in this game.
Depending on the outcome it will either be in favor of your decision or not, this will also give you points in various personality traits such as romantic or pragmatic depending on what you choose to say and these points will net you bonuses to your personality and other skills. Also it can be quite possible to make the two characters hate/love each other in the end.
This went on far longer than i intended so i will end this here by saying that the game is great and hopefully it will stay as amazing as it is now throughout the whole game (currently playing through it). Also, im Swedish so my english and grammar is not perfect, apologies for that.