Comment It's a solved problem (Score 1) 305
Teach the player early on what the difference is between a openable door and a purely aesthetic door. Some games (particularly ones with a sci-fi aesthetic) often use doors with green accents to represent openable doors and red accents for permanently closed doors. Some games like Deus Ex often have the door slightly depressed into the wall to signify a usable door, whereas purely aesthetic (closed) doors are generally flush with the wall.
Provide a different set of door types very early on in the game and the player will learn very quickly which doors to try and use. After a while it becomes automatic. To be honest I'm surprised this is seen as a problem anymore - I've never really thought about it for years. Maybe the next generation of game developers don't know much from history.