In addition to the basics, many conventions and 'best practices' have formed over the years. These are common throughout MUSH's and many have spread into the wider roleplaying community.
Show, Don't Tell Edit
A general rule of writing that is just as applicable in roleplay as in literature; it is far better to show the audience something than to merely tell them about it - whether or not something is impressive, frightening or beautiful is very much in the eye of the beholder. Describe the elements which might add up to those impressions, rather than just insisting that is what someone should take away.
Be descriptive, without waffling Edit
The longer a piece of writing is, the more likely your audience will have forgotten what happened at the start by the time they get to the end. It is obviously desirable to give them enough information to paint a vivid picture in their mind's eye but too much will actually make it unclear and difficult to visualise. Try to find a balance between brevity and detail.
'Power Posing' Edit
It is considered bad form to dictate the actions, or reactions, of characters you do not control - you may well throw a punch at someone, but unless it is vital that the result be included in YOUR pose, allow them to present their character's reactions. Sometimes, for the sake of brevity and narrative flow, you might ask a player's permission to pose a certain outcome on their behalf, but it remains up to them.
'Thought Posing' Edit
Poses are actions, they're used to describe things available to the five senses. Unless someone present is a telepath, they cannot hear what your character is thinking or intends. You can still give indications to your state of mind, but try to convey them through non-verbal cues. A bored expression, day-dreaming, yawning etc are all preferable to thought-posing being bored. This includes giving out information in descriptions that could not possibly be discerned from looking at someone.
Ask permission to join a scene in progress Edit
While most scenes are conducted in publicly accessible rooms, they can represent very large spaces indeed, and people often make calculated decisions on what to do and say based on who is present at the time. It is good form to ask if its alright to join a scene that is already on-going. It'll very rarely be refused, and you'll earn a lot of brownie points for being considerate!
Actions in descriptions Edit
Descriptions are used to convey an image and some basic sensory information to other players. Unless you're prepared to always take a particular action when someone looks at you, its best to avoid including reactions to being viewed in your description. They're unlikely to always apply.