Strong Feelings about Closets

I have strong feelings about closets. 

Closets are underused. Every room (and many corridors/hallways), above or below ground, could have a closet. 

Closets are always self-justified; the GM never needs to provide a reason for "why" the closet is there; players never question the existence of a closet. 

Closets are great because:

1. they have the mechanics of a door--they can be locked, trapped, bear strange labels or messages, be secret portals, lead to secret passages, etc.

2. they have the mechanics of a chest--they hide and contain something and must be opened to see what's inside

3. unlike a chest, there is no implication that what's inside is valuable to someone--it's equally plausible that the contents are valuable, mundane, or completely empty

4. they're a great way to provide background info, foreshadowing or clues to players--the contents of the closet (clothes, etc.) give clues as to who is nearby or what the area is/was used for

5. they are a great way to plausibly give "resupply" resources (food, kegs of water, a quiver of arrows, etc.) to a party, or fun, random trinkets to PCs, trinkets that otherwise wouldn't fit the story--hang clothes in the closet, and put the trinket in a pocket

6. endless storylines/drama/humor/irony related to someone/ something "finally coming out of the closet"

7. related to item (3) above, closets provide one of the best ways to actually implement a good "d100 List of Random Items" in your world, because anything can be in a closet

TIP: Always distinguish closets from regular "doors" when describing them to players. Maybe closet doors are "narrower than the other doors you have seen in here, you know, like a closet," or, "unlike the stone doors in here that have led to corridors or rooms, this door is made of wood and appears less substantial," etc. This makes closets a distinct element of the setting, which creates additional variety, interest, tension, mysteries, threats, opportunities, etc.