This one might be more a bit more contentious. But I think the action, bonus action system is really bad. In its essence it's pretty simple. But it causes much more question and incomprehension from new players (in my experience, of course) than almost any part of the system. It works. But it could be much better I think. In a game where one of the most, if not the most impactful element of balance is the amount of actions that both sides have (action economy), it's a weird concept to have a bonus action that you can't really use, except in situations where you have an ability that calls for it. And it feels really bad not to use it because you can't? Like, you really ought to take something you can cast as a bonus action.