So right now, reactions is handled on a "Toggle on/off" with a first come first reacted to way.
This works for attacks of opportunity, but DnD 5e has so many reactions you can take that won't really suit the current system.
Some iconic reaction spells like Shield and Counter spell won't be very worth while if they automatically go off against the first attack or spell that is used. Reaction is the last of the "action types" like Action and Bonus Action. It should be it's own little mark at the bottom of the screen, and using it should be a choice.
I think the only usable system for reactions is actually a "pop up" window, where you can select which characters should react to a triggered event. If a spell is cast, anyone who has counter spell, a spell slot AND a reaction available should be listed, and I can select the one to cast the spell.
With shield the attack should be shown if it hits, and we should be allowed to choose if we want to use shield if we have the spell, the spell slot and a reaction available.
And there's lots other opportunities for reactions. Sentinel feat is a very popular feat that uses your rections too
Now I hear people saying "But it slows down combat", but does it really? Combat is currently poorly optimized, and the animations are slow.
A poorly optimized combat shouldn't stand in the way of implementing 5e rules correctly. The poor AI that get's stuck all the time + an option to speed up movement, leaves more time for us the player to do things in combat, like our actions, bonus actions and reactions.
