A room description brings life to an area. It tells part of an overall story, while also depicting what the player sees when she/he/it travels along the highways and byways of your land. The theme throughout the area should be cohesive, a smooth blending of chapters, inviting the player into your tale.
It's the same with a mob description. A mob is a real person (in a MUDdy kind of way) with a history, and a reason for being in your land. The description and armor/equipment you provide the mob help 'flesh out' what would otherwise just be a stick figure, useful only to gain points or objects.
There should be a reason for each room to exist. It should not be a filler to make the area larger. Each room is unique, just as if you were walking down a street. Every few feet you see something different, something that identifies that particular square (or round or octagonal) footage.
When you are envisioning the room, close your eyes and ask yourself: What do I see here? What sights, scents or noises make this place special? How does this room relate to the rooms around it?
Remember that the time of day, and the weather, change. A room description which includes "The sun shines in through an opened window" would not be appropriate if it's actually night and raining in Arcanum.
It would be redundant to include descriptions of the mobs or objects in the room description. Save those for the mob or object description.
Try not to make the room description too long. Five to seven lines is a general average. Any less can be nondescript, any more can be spammy. Be thorough, yet succinct. Utilize keywords within the text to provide additional information. They are a bonus for the clever player, and can also provide clues that might otherwise be missed.
Never impose how the player is supposed to react. Descriptions like "You are filled with dread as you enter the room" may not be accurate for everyone. "A miasmic fog of dread permeates the room" can be equally descriptive, without making assumptions about how the player will feel. You are creating an image for the minds of others - allow them to determine how they will respond to it.
In most cases, the description shouldn't be too specific about the direction from which the player entered, especially if there are multiple exits. "Entering from the south," can be mis-leading if the player came from the north. If there is only one exit, it's not necessary to include "Head south to reach the waterfall", the player will figure that out on their own. Be sure that any directions provided are accurate and geographically correct for the area.
It might be appropriate to include visuals that provide continuity to the area. "To the far north the peaks of the mountains lie in silhouette against the horizon" could be important if that is the player's destination. Be sure though that it's something the player could actually see without pre-supposing x-ray or telescopic vision.
A thesaurus can be your friend. With a bit of imagination, repetition can be avoided. There are probably 16 different ways to describe "dark and cold", so expand the possibilities. Give the reading player a visual smorgasboard of variation.
Some builders choose to write their descriptions in a word document format first, so they can do a spell-check, and make sure that grammar is reasonably correct. This will save time later on, and helps ensure that the words flow smoothly.
The creativity and diversity you provide in descriptions can entrance and delight the players, making your area a place they'd like to visit often, like a favorite storybook, or epic poem. Have fun with it, knowing that your efforts will be appreciated by those who journey within.