I’ll admit, I tend to be a minimalist when it comes to any sort of in-combat action economy. Typically, no matter what the system is, I give players a move, an action (drink the potion, hack the computer, shoot the stormtrooper), and that’s it unless something odd comes up. And I’m always willing to toss even that out the window if it seems to make sense to do so.
This works with most games and in most groups I’ve played, but I could absolutely see it breaking down as sub-optimal, as it did when running The Lost City this past Labor Day. The PCs were fighting a gang of ghouls. The PCs got the ghouls in a narrow doorway, where only two could fight at a time. When the PCs got a bit worn down, they’d rotate out and let in fresh fighters.
Now, by strict 5e rules, the ghouls should have gotten opportunity attacks on them. What’s interesting, if you look at opportunity attacks, is that opportunity attacks are a subset of actions called reactions. And you only get one reaction until you take your next turn. So each ghoul would only get to make a single opportunity attack until their own turn returned.
More than that, if they used that opportunity attack, they’d not be able to take advantage of any other opportunities for either opportunity attacks or any other kind of reaction until their turn came around again. This means sometimes it makes more sense to ignore an opportunity attack if you know something better is coming along.
As an added wrinkle, some spells can be cast as reactions, meaning it really behooves certain spell-slingers to hold back and wait for the right opportunity if they think it’ll come up.
A lot of the same things can be said about bonus actions. Lots of things, but most especially class abilities, can give you bonus actions. You take a bonus action on your turn, mixing it up with your move action and your other action. However, as Mearls made a point of saying at GenCon, you only get a bonus action if something gives it to you and you can only take one. Mutliclassing and spells that grant you multiple bonus actions really only give you lots of options to pick from, but you still only get to pick one.
Which again keeps things simple and more interesting. You have to pick when to fire these special actions off, because you only get one each, and once you use ‘em you can’t again until your turn comes back around. This means a single player shouldn’t be hogging up lots of time by taking action after action on their turn. It may mean preparing to help someone through analysis paralysis, however. In the main, I think keeping things simple like this is good, but then, I like my combats short and sweet. As in all things, YMMV.