I dare to disagree Niara ...
Er.... with what exactly? I'm agreeing with you here, more or less - that it should be a setting that we can choose.
The option, as others here have suggested, for a full normal 5e implementation, and a complete reaction/decision point system that does what it's supposed to... and then there to be options you can choose, between a full 'ask at every decision point', down to 'never pause my game at all' and however many grades in between they feel is appropriate, is the best solution, since it gives everyone what they want.
If you want a game that removes the hassle of making those decisions yourself and will handle them for you in its best guess at what's best, to make your play experience as uninterrupted as possible, that's cool, and I absolutely support you having that option, but it MUST be an option only.
I basically wrote to agree with you that having it as a setting people can choose from different degrees of is ideal.
To Ikke,
Even those cases, as marginal as they are, are not acceptably fool-proof for always-on automation... you could have the game designed to always automatically use your OA if it's triggered by something directly before you in the initiative, but:
...Then it's an acid troll, or a fire elemental, or any number of other creatures that deal return damage on melee attacks, and you would, as a character on lower hp conscientiously not have taken the swing... oops, you're dead.
...Or it's a fresh-off-the-press larian adaptation that explodes into a large puddle of damaging poison when killed, and you would have preferred to let it run away so you could kill it from range... oops you're dead.
...Or you're a spellcaster and that creature that's moving away from you is doing so so that it can cast fireball on your whole party... and now you can't counterspell it because you automatically took your reaction as an opportunity attack... oops, you're all dead.
Outside of OAs, if it's a spell reaction, then it's never 'left over' at all, because it's a choice about whether you want to burn a spell slot or not... and that's NOT a choice they should EVER be taking out of your hands.
(Not sure what you mean by the second suggestion, sorry... could you explain what you're getting at there a bit more?)
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Definitely agree that there are far too many other far more tedious things bogging down the game that deserve attention...
