The actions are all extremely easy to understand. Standard action is your main action, immediate interrupts interrupt things while immediate reactions happen immediately after, as a reaction to the thing, move action is what you use to move, free action can be used whenever, and is essentially free, "no action" is barely even a game term, it is always explicitly clear when you do a thing that requires no action. They do what they say on the tin. Like I said, I've never seen anyone be confused by it, but I have seen many new players be confused by movement (why isn't it an action?) and bonus actions, and the language of things like using a bonus action to take the Hide action. 4e, worst thing is people forgetting they can use an immediate interrupt power. I've never had to stop and re-explain the entire action system to a 4e player.