There's a trope or two that says you shouldn't bother describing anything unless it's relevant to the story. However, in RPG's where the participants partially shape the story with their actions, any part of the world might become part of the story. How does one go about deciding the level of detail for descriptions of rooms/environments?
Usually when I describe things I have a few guiding rules around which I base my descriptions.
The first one is that there will be no more than three details in my initial description of the room/environment. I usually find 2 details too little and more details just confuse the players.
The second rule is that the description will be no longer than about 20 seconds. The logic is pretty much the same.
Combining these 2 elements makes the description detailed enough for me and my players but not too much. When they want to explore a certain aspect from the description I just give another 20 seconds chunk about it, thus giving the details just where they need them.
A typical description (for a dungeon crawl kind of game), thus, will look something like this:
You enter into a big room. There're a few chairs on the right which look a little bit rusty, while you smell a big stench from a pile of trash to the left. Between those 2, 3 goblins lie sleeping in their sleeping bags. What do you do?
Another one might look something like this (modern, this time):
You come to a huge square. Cars are going from side to side, making a lot of noise as they do so. A street performer is giving a mockery show of the queen, mimicking her voice and accent. A few policemen are running to the far end of the square, probably running after someone. What do you do?
One aspect that hasn't been discussed so far is the use of description as a pacing tool. The principle is quite straightforward.
For creepy scenes, or those where you are signalling that you are giving the players a breather you give longer, more detailed descriptions. For scenes where you want excitement and the action to move quickly you make the descriptions much more terse and to the point.
I have found that if you do this well, it is an excellent way of helping to get the flow of a session right, with periods of excitement mixed with relative rest.
In my experience it really depends on the game world and the tone of the campaign.
Modern or recent historical periods don't require much description at all, unless the game itself is focused on details. For example, for a run-and-gun action hero mercenaries game set in the 1980s a few references to movies or common tropes will do, because constant action and rapid pacing is at the heart of the game. Breaking that up with lengthy description would be disruptive. A meticulous, detail-oriented 1980s spy game that follows the Le Carre style and uses a d100 system with lots of specific skills and rules for noticing things would require full descriptions for every new environment, and that would not go against the spirit of the game.
Licensed game worlds like Star Trek or Star Wars tend not to require too much detail in descriptions because those worlds are so well known that shorthand will usually suffice for the broad sweep of description, except in situations where the PCs are encountering something outside the expected norms of the source material.
When the game world is further afield and it is more difficult for players to get a grip on what constitutes "out of the ordinary" or noteworthy, I'll sometimes spend a couple of minutes describing the scene and embellishing that description with background info (the rugs hanging on the wall don't just have sky runes on them, they're obviously indicators that the owner is an Rune Priest of Orlanth).
You could look at what writers do and do a similar thing. However, the main point to get is to let the player's mind fill in the gaps. For example which of the two below descriptions brings a more vivid image to your mind?
The car interior smelled as a cross between an ashtray and a fast food kitchen.
The room is littered with discarded fast food wrappers, half eaten chips, and cigarettes butts. It smells pretty bad of stale tobacco.
The trick (and it is a hard one) is to learn what works for your players so ask them for feedback.
It depends on the game and players' expectations.
Two things make details into play traps - if you choose not to play that way, as a group, and agree to not play that way, details can be fun additions to play and not confusing.
The "Gotcha" World
If the players expect the whole world to be potentially adversarial in an unrealistic, paranoid fashion, every detail becomes something to obsess over. This is your classic deathtrap dungeon, the old school game in which all 4 walls, the ceiling and the floor happen to all be living monsters, etc. This could also be the jerky GM who is out to get the players.
If you don't run that game, suddenly details are... fun interesting things and not potentially a life or death piece of info every step of the way.
If the players expect the only way to "play the scenario" is to try to pick up secret clues about what they "should" do next, then every detail becomes something to read into and try to guess what the GM is REALLY thinking.
At the worst type of this play, the players can expect their characters to suffer some form of punishment for failing to "go in the right direction" or at the least, to waste play time playing "Guess what the GM really wanted you to do" instead of simply having it given to them as a cut-scene.
Again, agree not to play this game and the players can just assume details are exactly what they are, and not secret messages about how to advance the plot.
One you get past those two things, the level of detail that's appropriate depends on the genre and what is entertaining - one genre might gloss over a detail in one setting and go in depth in some details in another - just as much as a movie or book might give you a sentence about one thing or give you paragraphs on another based on how much they want to "zoom in" on it.