To summarize, it looks like there are a few distinct suggested implementations of reactions. Let me know if I missed any (significant) ones:
1.) Automatic Use of 1st Eligible Reaction - i.e., BG3's current implementation.
E.g., AoOs are a toggle, as are things like Riposte. Hellish rebuke has to be pre-cast on your turn, but isn't cast against a specific enemy target.
Pros: simple, quick Cons: huge loss in control over your characters, potential waste of resources and/or suboptimal use of your reaction
2.) A Preaction system where you cast reactions on your turn, selecting eligible targets and conditions for activating.
E.g., Uncanny Dodge could be manually set to only activate if you take >10 damage. Shield, AoO, Counterspell would be cast only against certain enemies, and Counterspell could be set to only activate for level X+ spells
Pros: more, although still not perfect, control over your character Cons: potentially unwieldy and tedious. The UI would have to be very good, and you'd still probably spend a lot of time fiddling with reaction triggers often.
3.) Prompt-me System, where the game slows and/or pauses when you have an available reaction(s). You then have the option of using an applicable reaction(s) or not.
This is the closest representation of PnP Reactions. See @The Composer's suggestions earlier ITT.
Pro: Full control over character Con: The game would stop (or just slow down briefly) every time any character could take a reaction.
4.) QTE-like Reactions. You can pause the game at any point to activate a reaction in response to some incoming enemy action..
Pro: Full control over your characters
Cons: This seems difficult to implement and possibly frustrating to use. It is similar to a full-game pause which Larian seems reluctant to implement. Also, there'd have to be enough time to react to all circumstances, and to make that happen it seems to me that gameplay/animations/enemy actions would have to be slowed. It adds a real-time element to a TB game.
5.) Reaction Overhaul such that all reactions are cast during your turn, and their effects are appropriately changed to reflect this.
E.g., BG3's implementation of Cutting Words was changed from being able to subtract a Bardic Inspiration die from a single enemy roll (that you already knew the result of!) into an ability that subtracts the BI die from ALL rolls an enemy makes in the next turn. It has lost the power of being able to negate one specific attack in exchange for the *chance* to affect multiple opponent rolls.
Pros: Everything is cast during character's turns, and abilities now have appropriate power levels for the risk of lost resources/wasted reaction. Effectively, this solution turns characters' reactions into something more similar to the Bonus Action, where BG3 characters will now take 3 actions every turn - Action, Bonus Action, (P)Reaction Cons: Requires Larian to rework every ability that uses your reaction, and it's unclear how certain abilities like AoO and Counterspell can be appropriately reworked. It also makes reactions less impactful, turning powerful "use at a critical moment to negate the enemy's action" abilities into more diffuse "apply a debuff to the enemy" type abilities.
Obviously, these solutions could be mixed or matched where appropriate. The 3-way "Prompt Me / Automatic Use / Off" system is a mix of #1 and #3. BG3's system is already a mix of #1 and #5, where they've only made modifications to *some* abilities like Cutting Words.