Other examples of lower level reactions currently not in BG3 (as in ones you would have access to by level 4 among the currently released classes):
- Shield: A level 1 spell that grants the caster +5 AC against an incoming attack, with the AC boost persisting until the start of the caster's upcoming turn.
- Protection fighting style: An option for Fighters and Paladins wielding shields. If a party member standing next to them is being targeted by an attack, and the character with this ability is currently wielding a shield, they can react to impose disadvantage on the incoming attack (think of it as the shield wielder trying to block the attack directed at their ally).
- Ready Actions: Instead of using any main action attack, ability, or spell, a character has the option to hold their action until the condition to discharge the action is met. The simplest example is if Lae'zel is unable to reach anything to smack with a melee attack during her turn, so you can have her ready a swing for the first enemy that comes within melee range during their turn. Right now in BG3, you're usually either just going to end turn without doing anything or use some kind of projectile attack instead, which Lae'zel isn't nearly as strong in.
You also normally get to control when you want to use an attack of opportunity, instead of immediately using it against the first enemy that leaves your zone of control if you are wielding a melee weapon. But again, it is worth noting that you only get one reaction per turn, so a Wizard probably wouldn't want to use an opportunity attack if they may want to save it for an emergency Shield instead.
Right now, BG3's handling of the reaction system is to either ignore/not implement many of the abilities utilizing that concept, automate them to immediately use them against the first thing that can trigger the reaction, or change it into something that's pre-cast instead. It's also worth noting that enemies should be able to use reactions against you as well, so properly implementing them won't mean that the game will get easier.
It's really not an exaggeration to say that omission of a proper reaction system is basically missing 1/3rd of the combat system, and it consists of the majority of defensive options that exist in the overall 5E system too. Right now, it's not that much of a major thing, but the long term implications for the overall balance are rather dire.
Not sure if this possible in BG3 (yet), but in PnP rogues can make a sneak attack during an AoO since AoOs happen during the enemy's turn. So effectively they gain two opportunities for a sneak attack, once on their own turn and once on the enemy's.
