Door

Door is a dungeon feature. It can be just togglable obstruction of view and movement or a dangerous game object.

It is common for doors to be locked, requiring one to pick the lock or break the door or use a key to open the locked door.

Some doors are secret doors and cannot be seen until searched for and found. Monsters may or may not be aware of these doors.

Uncommon uses of a door:

In ADOM they are a potential trap location.
Caverns of Xaskazien uses them as obstacle blocking non-intelligent and weak monsters. There is no open command, one can just walk through.
In Xenocide plastic doors have a window which can be seen through.
In Rogue doors are always open and cannot be closed.
Doors with demons bound into them or actually consisting of a bound demon could be used to create a more formidable obstacle.
Doors can be jammed and not openable the normal way, requiring them to be opened by brute force. This can slow down the player or a monster. Moria and Angband allow a door to be deliberately jammed with iron spikes included in the game for that purpose. A door can thus be used as a barrier. Pretty much nobody ever bothers, however.
Doors could come in different sizes, restricting who can move through them.
It could be possible to try to listen at doors or peak through a keyhole to learn about the room beyond.
Doors could be a source of wood.
Doors could be edible for certain monsters.
Doors could be made of different materials, with different properties.
In a science fiction setting, doors may be automatic or have various opening conditions based on amazing technology. Doors in the Xilyl Hive may only open if you are a Xilyl or have an appropriate means to override the technology.
Some doors may only be able to be opened from one side.
Secret doors might require finding and operating some control mechanism to open.
Opening a door can be a source of noise and alert monsters to your presence (or alert you to a monster opening the door). Crawl has some squeaky doors and BOSS has doors with alarm.