In most other games pressing a single button instantly interacts with what you're looking at.
In ARMA by default "Space" once brings up a menu and twice it executes one action.
However this has been criticized because the action menu usually contains some complex actions such as changing weapons, magazines or deploying and activating explosives. As such when players approach a door and attempt to open it they can accidentally start changing weapons or detonating bombs simply because the door was slightly out of reach. Because the interactions are not visible until you open the menu you will have to open the menu and carefully select an action to make sure you do the right one which is counter intuitive and as the situation around you changes currently a lot of options have the potential to jump around a bit.
Adding an “Interact” button that only interacts directly with what you’re looking directly at would greatly increase the speed and security of common interactions such as opening doors, entering vehicles and so on.
“Interact” would only be used for the same actions that an action button would be used for in other games while these common interactions are also removed entirely in the action menu leaving a cleaner and more understandable action menu as well.
By default I suggest “Space” to common interactions.
On a final note having “Interact” also confirm action menu interactions that are selected normally would make the controls smoother and work as I think they do now with the default bindings.