Hell Is Not Other People
To make this short, Hell is not other people, because Hell is people like yourself. In other words, hell is yourself.
The fact that everyone refuses to leave despite the door being unlocked is basically a direct statement on a person's need for other people, regardless of who they are.
The unlocked door itself symbolizes the potential for change, while the people like ourselves in the room symbolize the reflections of our negative traits we see in others, whilst the two symbols combined are themselves a symbol of the human potential to escape from one's self, from one's already defined qualities, into something different, and possibly better.
In short: the room is you. The other people in the room reflect your negative traits that you see in others, and you within the room represents your tendency to self-aggrandize and cast your worse parts onto others, as opposed to taking responsibility for them yourself. The unlocked door represents the idea that "no man is an island."
Reply: While an interesting interpretation, the person above has obviously never read the story, because as far as I recall, the story is about three people being LOCKED in a room in hell. (The door was never unlocked as you're saying.) (P.S. stringent binary left/right thinking when it comes to values is bullshit..) Source: https://archive.org/stream/NoExit/NoExit_djvu.txt (CTRL+F, look for "locked", lol)