I don't know about the 3 second thing
Yeah, I also don't like the 3 second suggestion, but like the rest of the idea. Why limit it to 3 seconds? It is turn-based, after all. If you wanna enforce time-limit on reactions, you should do it for actions, too. After all, one D&D combat round lasts for 6 seconds.
I think Reactions are one problem where they *can* make everyone happy with not that much effort. For those who prefer to play braindead and have mistaken this game for a 3rd person shooter, leave them the option of using reactions as they are, or, hell, give them the option to remove the concept of reactions entirely. Dumb it down for them even more. Remove bonus actions for them if they want, wouldn't want to pile too many options on them. Might get in the way of that sweet, sweet action. For those of us who expect a turn-based game to take turns between player and enemy actions and reactions, please implement it properly. Because as it stands now, Reactions feel like a half-baked potato and I'd preferred them taken out completely to what we have now.
