Instead of creating one 'main' character as in most RPGs, or creating an entire party that is relatively faceless as in Icewind Dale or Legend of Grimrock, you create two main characters. While this is intended to service a multiplayer campaign, it creates an interesting situation for single player. RPG players tend to let such created protagonists act as vague ciphers for themselves, but having two at once inspires some level of contrast between the two. This inspires a bit of actual role-playing. For example, I created a woman and a man and decided that the woman would be a sarcastic, mercenary warrior and the man would be a rigid, by-the-books wizard and they'd share a sort of buddy cop romantic comedy opposites attract sort of thing. Unfortunately, this potential is squandered outright as the game puts a lot of effort into letting you customize your characters' philosophies and basically none into their personalities. The quandaries your characters are put in are things like "a healer has one cure but two patients" and "a man is stealing food for his hungry family." The decisions you can choose from can inform your characters, but without being able to establish them it's a bit wasted. It's like if you knew George Costanza's views on abortion but nothing else about him. What's worse is that disagreements between the protagonists are literally solved with rock-paper-scissors.
That disappointment aside, the Divinity puts its best foot forward by starting with a murder mystery in the city of Cyseal. This town has a lot going for it-- a lot of hidden areas, several sidequests (that often intersect with the main questlines) and multiple ways to accomplish tasks. Thieving skills can help you find evidence, a perk that lets you talk to animals can get you unique information, and speech checks can narrow down your suspect list. It also ties into the personality/multiplayer system somewhat since your other half might disagree with stealing or digging up graves. Leaving the city, where monsters must be fought, can lead to more clues. All in all, the opening in Cyseal is by far the best part of the game, and one of the best RPG experiences. It's unfortunate, then, that once it's over that style of gameplay goes with it. There is a second, smaller-scale version of it much much later in the story where you infiltrate the bad guys' settlement in search of prisoners. That bit has a nice cat & mouse feeling to it, since you can help the bad guys with their problems for short term gain, or rat them out to their commander and have them executed in order to make the big climactic battle a bit easier for yourself. There's also the danger that the prisoners you're looking for can be executed.
Aside from those two areas the rest of the game is basically roaming the countryside getting into fights and delving into dungeons. While it's a shame to see the amazing gameplay of Cyseal go, the combat is really enjoyable. Like much of the adventure portions, the combat is orientated around an Ultima-style interaction with the environment. To use a fireball spell you want to direct the AoE splash damage away from your party members, of course. Additionally, however, the fireball will now leave a firey patch on the ground that will cause damage and a status ailment that drains health. As a result you also do not want to shoot a fireball where your party members are going to be (and, hopefully, where your enemies will be). What's more, many of the elemental effects interact with one another. If fire touches poison clouds an explosion will occur, damaging anybody nearby but also creating an area of thick black smoke that obscures line of sight. A patch of ice can be melted into a pool of water, which can then be electrocuted. It opens up even more as you gain access to spells that allow you to teleport things-- enemies, allies, and objects-- around the battlefield. Or abilities that interact with corpses. Or make it begin to rain. Is it worth it to electrify the floor to stun that monster if the other monster is healed by electricity? There are a ton of things to consider and a lot of room for creativity. A few bugs to drag it down somewhat. Something that happened a little bit too often for me was that enemies (especially bosses) would bug out and just sit there doing nothing. Thankfully the game has some sort of failsafe timer that ends their turn after about 30 seconds so it doesn't create a soft lock, but it's not a good look and waiting a full 30 seconds for the dummy to end its turn after just standing there is annoying. I also want to say that the final battle is pretty annoying since it centers around protecting an NPC that stands in one place, refuses to heal or buff itself (but will heal and buff other characters that don't need it), is explicitly targeted by the AI, and causes a game over if killed.
The main quest, like the city hub areas, is fairly open. I managed to accidentally sequence break numerous times (which speaks to the sometimes iffy nature of the signposting). I love that this can happen, but it's not always handled well. Doubling back to do things you missed can result in major characters reacting to things that they should be very familiar with as though they just heard of for the first time. "Heroes you must go, the villain intends to use the stones to revive the demon!" and then later: "Heroes, you must go to <place you've already been>. It seems the villain is using the stones for some nefarious purpose..." Stuff like that. It's a minor gripe, but if you're going to make the sequence breakable, you should write around it better. Although the game does enforce its sequence with its difficulty curve, which can be brutal if you try to paint outside of the lines. The enemies in Original Sin are strictly leveled, to the extent that facing a Level 15 monster with a Level 13 party is extremely difficult. The character progression system itself is very strict. Levels come slowly and the best spells are locked tightly away behind large point-buy barriers and a respec system that is more trouble than it's worth. Putting a few points into "Barter" at level 3 might come back to bite you at Level 19 when you can't get the Meteor Shower spell.
All things considered, if there's only one thing the game does poorly it's puzzles. There are quite a few of them and they tend to be very obtuse or devolve into pixel hunting for tiny switches. Some of them are just kind of unfair. An early example has you stuck in a room. "If you move the room will explode," your character is told. I spent a long time blowing up over and over trying to figure out the trick. It turns out that the "you" only applies to that specific character and that anybody else in your party is free to walk over and disable the trap. What the hell is that all about? The game also has a weird love affair with stealth sections, although only one is actually mandatory (and guess what: it sucks). These become easier to manage once you're stocked up with invisibility and teleportation spells and can cheese your way through, but I still don't get it. One thing I want to give props for, though, is that in spite of the large amount of traps and damaging floors, the AI pathfinding is extremely good about walking around danger instead of bumbling straight through it.
Divinity: Original Sin is a very very good game that could have been an amazing game had it focused on its strengths more. It's a large game with a lot of stuff, so my laundry list of things I wish they had done differently is pretty long. However, I highly recommend it, and is up there with Shadowrun: Dragonfall in terms of RPGs from the 'kickstarter" era.