What are the major rules differences between D&D 5e and Baldur's Gate 3

Question

The rules in Baldur's Gate 3 are largely those of the pen & paper RPG Dungeons & Dragons 5th Edition

I‘m coming to the game from having played a lot of pen&paper D&D, and the older Baldur‘s Gate games, which as I recall it were a bit more lose about the rules compared to the old D&D rules.

In any case, because for this one, the rules seem to adhere very closely to those for the tabletop RPG, both for actions, attack rolls etc in combat, as well as for character development, I‘ve been thrown off a few time where they differed.

For example, crossbows have no limit to number of shots per round in BG3 (which in the paper version is one of the balancing factors for their generally higher damage than bows). Magic items seem to lack attunement, which in the paper version limits how many strong items a character can use at the same time. Concentration spells that in the paper version often are only good for a minute (10 combat turns), or a short perid of 10 minutes often last for the whole day, as long as concentration is not broken (e.g. expeditious retreat).

What are the main discrepancies from the written 5e rules, where things were implemented slightly different in the computer game?

Answer 1

Raven Dreamer’s answer gives a good overview, but if you’re interested in an exhaustive list of changes, one is available via the independent Baldur’s Gate 3 wiki project. The main article covering changes to basic rules like initiative, healing, resting and more is “D&D 5e rule changes”, and there are also further articles for changes to races, classes, spells and feats. These articles all assume some familiarity with standard 5E rules. These pages are also updated when patches make further changes to the rules used by BG3 (especially ones which fix bugs preventing a rule from operating as it does in the tabletop game).

I would add that there are some changes that are not obvious but have a big impact on the game. As one example, initiative is not a standard Dexterity check, but instead uses a d4 (though the Dexterity modifier is still added). This makes it more likely player characters will end up next to each other in the initiative order, and such characters effectively act simultaneously, able to take their actions and movement in any order they like. Like many of the changes to a basic rule, this has many knock-on effects, such as modifying how many bonuses to initiative work (advantage on initiative rolls is generally changed to a flat bonus of +3 to +5, for example).

A few other significant changes which surprised me and affect power level:

• Ranges for weapons and spells have been reduced across the board (usually to 60ft/18m), and combat generally takes place in smaller areas with a lot of opportunities for vertical movement. Being at least 10ft (3m) above an opponent can grant a bonus on ranged attacks, and likewise being below them imposes a penalty. (By the way, the game can use metric measurements, using rounded approximations of 5 feet = 1.5m, and 1lb = 0.5kg. As an Australian I love this.)
• Cover is not implemented, but objects and creatures can block line of sight.
• Prepared spellcasters can change their list of prepared spells instantly at any time outside of combat.
• Ritual spells don’t take longer to cast, they just don’t use a spell slot unless cast during combat. Some higher level spells are now rituals, including speak with dead.
• As you noted, most spells and effects with a duration longer than 1 minute now just last until the party’s next Long Rest.
• Most potions can be thrown (as an action) in order to make them splash on the ground and apply their effect to multiple characters. This includes healing potions.
• By default the game has a “Karmic Dice” option enabled which prevents streaks of very high or very low rolls. This always acts in favour of the character or creature rolling the dice, and results in shorter more dangerous combat, since everyone is more likely to hit and do higher damage. This gives spells with saving throws a disadvantage vs ones with attack rolls, since the target rolls the dice.
• Humans and half-elves have revised traits to make up for losing their additional Ability Score bonuses (since everyone gets a choice of +2/+1 to any Ability Scores). This includes a “Civil Militia” trait which grants them proficiency in various polearms and shields.
• Spell components (including foci) are generally ignored, so a human wizard can equip a staff and shield and still cast spells. Being silenced (for example by the silence spell) does prevent spellcasting, which along with the vocalisations when characters cast spells suggests an implicit verbal component, but a small number of spells are explicitly called out as being usable while silenced.
• The AC bonus from a shield equipped in the melee weapon set is applied even while using a character’s ranged weapon set.

There are many more, but those give you an idea about the extent of the changes.

Answer 2

First a disclaimer: I do not expect this answer to be exhaustive. BG3 is a large game, with a lot of content. In part because of that, I will be focusing on the broad categories, rather than individual miscellanea.

Character Options

D&D 5e has been out for multiple years, and has over a dozen sourcebooks. While Baldur's Gate 3 (BG3) has a large variety, and covers most of the "traditional" fantasy options, there are gaps between what is available as a character option in the game versus the tabletop.

This encompasses such varied categories as race (there are Elves, but no Changelings), subrace (where e.g. Githyanki is available, but not Githzerai), sub-class (e.g. "Trickery" domain, but not "Twilight"), feats ("Durable", but not "Elven Accuracy"), spells ("Fireball", but not "Greenflame Blade"), clerical gods ("Mystra", but no "Cyric") and even class (sorry, Artificer...).

There's also the fact that the game caps out at level 12 - generally speaking, there are no 7th level spells at all, and other high-end class abilities are likewise not implemented.

Multi-classing, however, does not have ability score requirements at all.

The long and short of it is, if the character option is unavailable, it is much more likely that it is never available, rather than simply inaccessible.

Mechanical Differences

Some spells and abilities are simply different in BG3. "Spell Sniper", for instance, does not allow free range pick of any qualifying cantrip, but instead gives a 4-spell list to choose from; A Paladin's Lay on Hands isn't the flat 5 HP / level "pool", but rather a charged ability which derives its potency from class level instead; A cleric's "Spiritual Weapon" spell summons an actual entity that can be damaged (and destroyed), but no longer requires the cleric to spend their bonus action to command it to attack.

This is the category that is going to differ most wildly for someone with prior experience with 5e D&D.

A special nod towards shapeshifting (wildshape et al) -- many reactions are unavailable while shapeshifted, even if mechanically they would work fine (such as Tempest cleric's "Wrath of the Storm").

Weapon Abilities

Martial characters in BG3 have slightly more options when it comes to fights -- each weapon they are proficient with gains access to one or more "weapon abilities" that can be used in place of a regular attack (or in addition to). The closest analaogue would be Battlemaster manuevers, with each one being usable once / short rest.

Action Economy

The action economy is quite different. First, "Shove" is now a bonus action, instead of being tied to the "Attack" action (though it's still based on strength -- good luck, Wizards), as is "Jump" (instead of being handled as part of movement).

As your other question observes, there is no "loading" penalty on crossbows (and Crossbow Expert is tweaked to provide a different bonus instead), which leads to everyone being able to make bonus action attacks with an off-hand hand crossbow or light weapon (even when you haven't taken the "Attack" action).

There is no rule that limits casters to cantrips if they cast a spell using a bonus action.

And lastly, falling prone immediately ends your turn.

Resting

Long resting requires an in-game resource (Camp Supplies) to trigger, but can be triggered essentially any time outside of combat. Short rests are more limited -- only two are allowed between long rests, and they always restore exactly 50% of your HP, no hitdice required.

Certain item effects and spells last "until your next long rest", rather than a duration in days or hours. (such as Longstrider, or an Elixir of Hill Giant Strength)

Conclusion

Ultimately, I think most of the "Gotchas" come from the "Action Economy" heading -- other differences tend to be much more readily apparent from their descriptor, whereas these exceptions are never explicitly spelled out (after all, BG3 doesn't have a "PHB" of its own describing the rules!).

I think in general it is probably ideal to not worry overly much about whether or not BG3 is a perfect translation of D&D 5e into the virtual space (as this answer indicates, it isn't), but rather enjoy it on its own, for what it is.