Many will likely compare Divinity: Original Sin to Baldur's Gate, and it is a fairly apt comparison as the best way to describe Divinity: Original Sin is as a modern-day iteration of that franchise. That is only to help with familiarity, it would not be fair to Larian Studios to just call this an updated Baldur's Gate as it does some incredible things all on its own.
The first thing that you will notice upon playing Divinity: Original Sin is the fact that you actually create two separate characters that function as the two main characters of the game. At first I thought that was interesting, and then I thought it was brilliant because of what Larian Studios did with the concept.
There are literally hundreds of different decisions (more on that later) to make throughout the game and with each one you have a chance to choose dialogue options between the two different characters, which means that you can essentially apply two different personalities to each and see how they clash within the game. If there is no compromise the game has the decision go down to a rock-paper-scissors game to see whose choice gets to be the one that goes through. It is funny sometimes too as the characters you interact with might comment on the fact that the two heroes disagree. It became one of my favorite parts of the game as I basically created two characters from some short stories I have written and had them interact.
Even when you come across an event, or read a book, it may pique your characters' curiosity enough that the two will have a conversation. This can be quite interesting as well because each character can express their feelings about the situation and have the other react to it. This of course gives the illusion that you are more involved in creating the character than you actually are, but it is scripted reasonably well enough to come across as fairly genuine.
The choices you make in dialogue, and the choices they lead to in a game (like arresting or killing a character), all have effects on your characters as well. Depending on the decision a character makes they put points toward a trait. For example, on one end of a bar is "Romantic" and "Pragmatic" on the other. Each trait then gives your character some kind of bonus to a stat or skill.
The way quests are constructed and the way you get to make decisions is probably the strongest factor behind Divinity: Original Sin. For example, in one quest a person wanted me to kill another person and bring back an amulet as proof of their death. In that I could have killed the person and retrieved the amulet, just stolen the amulet, searched for evidence against the person asking me to kill the other, or outright kill the person that asked me to kill the other. That is just a basic quest to understand the dynamics Divinity: Original Sin has. Other quests are far more complex and can be solved in many different ways. You can even choose to keep an item, like a weapon, someone asked you to retrieve to end the quest (though you won't get the benefits from turning in the quest).
As for your characters, you have the choice between basically eight different "classes." At any point in the game you can add a skill to your character which opens them up to the possibility of learning skills from one "class." For example, I have a mage-like character in my party who had some ice spells that were useful, but no fire which also is useful. So the next time I had the opportunity to learn a new skill I put it in "Pyrokinetic" so he could learn a fireball spell.
The way you learn spells/abilities can be frustrating. You can either come across them as drops, find them in the world, or purchase them from vendors. They are books that you then have to use. So, right now I have a bunch of books I haven't and may not use because I don't have anyone within that particular class. So it is possible to be stuck with some similar abilities for a while. Though in the game are numerous scrolls you come across that help combat this as any character can use a spell scroll (only one time though), so you can spread out the scrolls and temporarily boost a character's choice of abilities.
Larian Studios boasted of the amount of customization you can put into a character - they were right to boast. I'm sure many immediately think "cool, I can make my character look however I want." Well, not really, the visual customization is fairly average. However, by mixing those "classes," gaining certain traits, putting points into various abilities, and putting points into talents, you can make a unique character. Talents give a wide variety of benefits, from being as simple as boosting a stat, or giving a character the ability to heal when they stand in blood on the ground. Combining and changing all of those things allows you to make a really unique character.
Of course, in some ways that falls flat as some systems that are in place are fairly basic. Like the "sneaking" or "pickpocketing" mechanic. They are exactly what they sound like, but fairly basic overall. With a game so packed full of things it is not surprising that some fall short.
As for combat, you can have up to four people in your group/party at any one time. The first thing to know is that it is actually pretty difficult (I had to play on easy sometimes just so I could keep going). Positioning plays a huge role as there are numerous AoE spells that can really hurt and there are obstacles to consider in how enemies may or may not be able to attack you. Many times spells/arrows are blocked because there is something in the way. None of the abilities or status effects should be unfamiliar to anyone that has played an RPG in the past. Things like poison, blinded, burning, bleeding, etc.
The abilities themselves are not what is interesting but how they interact, how you use them, and the way you move about during combat is interesting. Each turn for your character you have action points that get used up by movement, using an item, or using an ability. You can save up some one turn to have more next turn in case an ability takes a lot of points. Knowing who to target and being careful with your action points will be what you spend most of your time thinking about.
Just reading that you may think that there isn't a whole awful lot to the combat. What I covered was just the basics to give a feel for what it is like. Divinity: Original Sin's combat is full of intricacies and subtleties. A character can slip and fall on ice to be knocked down for a couple of turns, you have to watch out for oil on the ground that can be set on fire, you can use a spell to cause rain to put out fire, and much more. You also have to be constantly aware of friendly fire so you can either use it to your advantage (having enemies hurt each other) or have to watch out for your companions.
The overall story is something that fits in to a fantasy setting, but isn't anything to get super excited about, just something to string you along from area to area. Most of the effort, and what is done very well, went into writing the world. The world is very interactive, each person you come across can be bartered with and talked to. When you come across a locked door and don't have the key, you can pick the lock or just bust it down. That can play a huge role in quests as well, as sometimes you need to get into a door to get evidence against someone or retrieve some item. The interactive world directly plays into the writing of it. Sometimes interacting with characters would get frustrating because you have to navigate through dialogue until you find the one thing you were supposed to select to get from them what you wanted, like turning in a quest.
Instead of getting immersed into a story, you get immersed into the interactive world. There are some interesting quest arcs sprinkled throughout, which as always can be solved in many ways, so the overall writing is quite wonderful.
One of the places that Divinity: Original Sin is lacking is the sound. Most of the sounds are alright, but some of the voice acting is fairly subpar. Also, it is quite often that the only thing you will hear are the various characters around you talking, repeating the same thing often. A lot of times when I was looking at a menu or inventory I had to move my screen away so I wasn't overwhelmed by everything.
The visuals are very pleasing, very reminiscent of something like Torchlight, but an updated version. A lot of the areas look very good and the art direction is decent, but for the most part the environments you find yourself in are familiar and nothing new. Still, Divinity: Original Sin makes what we may have seen already look quite good.
Overall, Divinity: Original Sin is one of the truest RPGs ever made, not only one of the truest in a long time. Larian Studios deftly put a world in place for you to just interact with in any way you see fit. The only places that Divinity: Original Sin lacks are those that do not facilitate player autonomy. Their greatest efforts were put into parts of the game that allow for you to do whatever you want.
If that doesn't excite you as a gamer, I don't know what will. There is so much more that just couldn't fit in here and you will have to experience for yourself (like getting the ability to talk to animals). Honestly, it's probably fair to say that this is one the best things to come out of Kickstarter.