Characters
The first thing you do in a game of Baldur's Gate III is create your character. At some point in the future, you'll be able to choose one of the pre-made "Origin Characters" (just like in Divinity: Original Sin 2), but for now those characters are only available as companions, and you have to build your character from scratch.
Larian is using the Dungeons & Dragons 5th Edition ruleset for Baldur's Gate III. That means characters are defined by their race, class, ability scores, and skills. You're also given some cosmetic choices for your character, including a wide range of appearances (so if you want your human character to look like Papa Smurf, you can do it). I can't tell yet if gender is cosmetic or not. I don't remember anybody commenting on it, and if companion romances exist at all, you can't get very far with them in Chapter 1.
The races in the game include drow, dwarf, elf, githyanki, half-elf, halfling, human, and tiefling. Each race gives you a collection of bonuses and proficiencies. For example, drow gain Superior Darkvision (which allows them to see well in the dark), proficiencies with rapiers and shortswords, a bonus to perception checks, +1 charisma, +2 dexterity, and more. Meanwhile, humans gain +1 to each ability score, which is boring but useful.
Classes currently include cleric, druid, fighter, ranger, rogue, warlock and wizard, but I've seen game files for bard, monk and paladin as well, so this part of the game appears to be a work in progress. Chances are, when Baldur's Gate III eventually ships, it'll have all of the 5E base classes. Each class has at least two sub-classes, which you either pick right away or at Level 3. Multi-classing is not yet allowed. As with races, classes give you a variety of bonuses. For example, fighters gain proficiencies for all weapons and armor, plus bonuses to strength and constitution checks.
Ability scores are the same as you've no doubt seen in countless other D&D games (and their copycats) -- strength, dexterity, constitution, intelligence, wisdom and charisma -- and they do about what you'd expect. For example, strength improves melee damage and attack rolls, and increases carrying capacity. You get a certain number of points to spend on ability scores when you create your character (no rolling for scores, at least not yet), and you get two additional points every four levels. Ability scores can't advance beyond 20, which limits the amount of min-maxing you can do.
Skills include things like history, persuasion, religion, and sleight of hand. All characters have access to all skills, but they don't receive skill points or anything like that. Instead, their ability scores give them a base value in each skill, and then they choose a handful of skills (based on their class and "background") to become proficient in. The proficiency bonus starts out as +2, but then grows as characters gain levels.
As an example, the sleight of hand skill (which is used when disarming traps and picking locks) is based on dexterity. With a dexterity score of 16, you start out with +3 in the skill. If you're also proficient in it (say, if you're playing a rogue), then you add your proficiency bonus to the score for a total of +5. All skill checks use a d20 die, so +5 means you have a 25% better chance to succeed over somebody who is at +0.
I didn't much like the skill check system, and the example in the previous paragraph is the reason why. If focusing on a skill only gives you a 25% better chance over someone who is totally inept in it, then something is wrong. Worse, it means that any character can succeed in any skill check, if you don't mind save scumming. This is an area, I think, where Larian following the 5E rules exactly is a mistake. They should adapt the rules to work in the medium they're using. You can't repeatedly reload your game in a tabletop setting, but the temptation is great when playing on a computer / console, especially when your d20 roll fails to beat a 5.
To offset d20 roll frustration -- at least a little bit -- Larian uses "inspiration points." These points can be gained in a few places in the campaign, and you can use each point for a second roll during skill checks. So if you fail to beat a 5 on your first roll, you can spend an inspiration point and try again. Of course, inspiration points are rare, so you can't use them all the time. You have to pick your spots -- which for me means I never use them because I'm always worried I should save them for someplace "really important."
In my latest game, I played as a druid. I gave him the "urchin" background, which made him proficient in sleight of hand. Since he also had 14 dexterity, that gave him +4 to the skill, making him about as good as any rogue in rogue-like activities. For me, that just felt wrong. Druids and rogues shouldn't have much in common. I preferred the 2E skill system in Baldur's Gate II where classes were restricted in what skills they could use, if they could use any skills at all. But then, admittedly, I'm old, and you whippersnappers should get off my lawn.
Campaign
You begin the game on board a flying mind flayer vessel called a nautiloid. You've been kidnapped by the mind flayers, and they've infected you with a parasite that will eventually turn you into a mind flayer yourself. Apparently, that's how mind flayers procreate. Who knew? But luckily, the vessel is attacked by a githyanki war party, and it crashes into the countryside outside of Baldur's Gate.
While you're exploring this region, you meet other survivors of the wreck -- all infected like you -- and five of these characters become available as companions. Unlike most RPGs, it makes sense for the companion characters to join you, because you all have a common goal -- to find somebody who can remove the ticking time bomb of a parasite from your head. And conveniently, the companion characters cover the basic classes -- cleric, fighter, rogue, warlock and wizard -- so it's easy to form a party of up to four characters, regardless of which class you choose for your main character.
The crash site region has a lot of variety to it. There's a druid grove where tiefling refugees are hiding out from goblins. There's a burning inn where people need to be rescued. There's a bog where a wicked witch has set up shop. There's an entrance to the Underdark, where you can pal around with a community of myconids. And more. In fact, there's so much stuff in the region that's it's a little difficult to believe, even in the context of a fictional computer game setting. It doesn't feel like a living, breathing place. It feels more like a greatest hits album.
There's a similar problem with the combat. Larian eschews trash fights, but to make the battles more meaningful, they've all but turned each one into a boss fight, complete with a bunch of crazy bosses. You can only reach Level 4 in the game right now, but you still encounter beholders, bulettes, harpies, mind flayers, minotaurs, and owlbears -- plus, of course, the wicked witch and a series of goblin bosses. I sort of worry that Larian is going to run through the entire Monster Manual before the end of Chapter 2, and then not have anything left for the rest of the game.
Of course, while you're visiting this smorgasbord of enemies and locales, you also meet a bunch of people who need your help. The quests they give you usually work pretty well, and there are always options for how to complete them. For example, at one point you meet a wounded tiefling hiding out from a group of paladins. The tiefling tells you that the paladins are really henchmen working for the demon she escaped from, while the paladins tell you that the tiefling is a cold-hearted murderess. So whom do you believe, and what do you do? This example quest is all about combat, but in other cases you can bribe, charm, deceive or intimidate your way to success -- assuming the Random Number Gods favor you and you win your skill checks.
The writing in the game is good but not great. There aren't any megalomaniacal creatures running around doing evil things just because they're the designated bad guy. Characters are given motivations, and everything makes sense. But while I've found the dialogue to be intellectually interesting, it hasn't affected me in any sort of emotional way. Every so often Larian tries really hard to tug at your heartstrings -- oh, no, a tiefling child was caught stealing, and a hardline druid wants to kill her! -- or they throw in a squirrel for comedic effect, but most of the time I didn't really care what happened, or if the people involved lived or died.
This extends to the companions, who aren't exactly personable. The more they approve of what you do, the more they talk to you, and the more you learn about them -- which presumably will lead to quest and romance options later in the game. But the companions add so little so far that I wouldn't mind dropping them altogether and creating three more characters from scratch -- only it's not allowed, at least not yet. Of course, the caveat here is that it's still early in the game, so maybe we're still in the getting-to-know-you phase, and the companions will bloom once the watering has been completed.
So far I haven't noticed any links to the first two Baldur's Gate games -- or, really, much that makes me think of the Forgotten Realms. Volo shows up briefly in the druid grove, but that's about it. Maybe later we'll see Drizzt again -- it's tradition, after all -- and get another crack at Twinkle and Icingdeath.
I'm probably making the campaign sound worse than it is. The creatures, quests, and locations are all solidly crafted, and the excellent voice acting (especially from Amelia Tyler as the narrator) really sells the dialogue. If Larian continues in this vein, then Baldur's Gate III is going to be just fine.
Combat
Baldur's Gate III uses turn-based combat. When a fight starts up, the characters involved in the battle are ordered using their initiative stat, which is derived from their dexterity. Then the characters take their turn, during which they can move, perform an action, and perform a bonus action, in any order. The amount they can move is based on their race. Actions are typically attacks and spells, while bonus actions are more utility-oriented, such as jumping and quaffing potions.
If one of your characters loses all of their hit points, then they're knocked unconscious rather than killed. This allows any other character in your party to "help" them and restore them to 1 hit point. You can also cast healing spells on unconscious characters to get them going again. If an unconscious character takes enough damage, then they finally die, and you have to revive them using a Scroll of Revivify (or possibly a Raise Dead spell later in the game when clerics are high enough level to cast it). Failing everything else, you can also pay to revive characters, but to do that you have to visit a specific location, which means you have to finish the fight first.
Because Baldur's Gate III is a Larian game, the environment plays an important role. There aren't as many elemental effects and barrels as in the Original Sin games, but they're still there. More importantly, height makes a difference. Characters on high ground have an advantage over characters below them, and characters on low ground have a disadvantage against characters above them. So half the battle sometimes is positioning your party in the best possible way.
Using my most recent party as an example, during the first round of combat, my druid might shapeshift into a bear (bonus action) and then move and attack someone with his claws (action). Lae'zel the fighter might cast Tyr's Protection (a bonus action available from her sword) to improve her AC, move and attack someone with her sword (action), and then invoke Action Surge (a once-per-battle ability) to gain another action and attack again. Gale the wizard might cast Misty Step (bonus action) to teleport to a good spot and then cast Magic Missile to damage an opponent (action). And Shadowheart the cleric might cast Survival Instinct on a character (usually Gale) to keep him from getting knocked unconscious (bonus action), and then cast either Bless or Bane (action) to improve the attacks rolls of the party or reduce the attack rolls of the enemies.
I enjoy turn-based combat, so perhaps I'm biased, but I thought the battle engine worked pretty well. You're given lots of options for what to do, and the enemy AI is competent. Enemies move to high ground when they can, they target your spellcasters relentlessly, and they know when to cast area-effect spells (or throw bombs) so they damage you and not their allies. The battles are also difficult, which is nice, but this might have something to do with being limited to Level 4 in the game, which you can reach about halfway through the available content. At Level 5, for example, fighters gain a second attack per turn, which makes them much more powerful. Larian will probably be tinkering with the difficulty until the game is eventually released.
Because combat is tough and characters can't memorize many spells yet, that means you need to camp fairly often to keep your party battle-ready. For some reason, there are two kinds of camps: a regular camp where you completely heal and memorize all of your spells, and a short rest where you heal a little and regain some abilities (like Lae'zel's Action Surge). The regular camp takes place on its own small map and gives you a place where you can store items, and it acts as your base of operations for the game. Interestingly, despite having a parasite in your head, you can camp as often as you want, and there isn't any downside. There isn't any time limit in the game.