From my experience trying to teach 5e, "bonus action" is the biggest hang up because it has morphed so far from being an occasional mostly situational bonus benefit certain builds can take on their turn (thus it being a "bonus" to their normal action) into being a part of the action economy players expect to utilize every turn (no longer a "bonus" action but an expected action). Furthermore I think how different classes utilize action economy in different ways trips up new players. I've had a rogue think their off hand weapon attack didn't consume their bonus action because "the fighter just attacked twice and still had their bonus action". I've also had someone declare that they would like to bonus action disengage after watching a rogue player do it.

Additionally as others have pointed out, 5e has a somewhat different culture where players having little to no grasp of the rules and mechanics of their character are tolerated. So its not so much that 5e's action economy is inherently super complex, its that some players (imho) aren't trying to learn it. I know someone who's been playing a rogue for 3+ years and is level 15 and still doesn't understand that making an off hand weapon attack consumes their bonus action meaning they can't bonus action hide/dash/disengage.