Be Original Edit
Wherever possible, aim for original content. There are already many sources which list details about the game, so imitating them just so we can have another page on the site makes little sense. Never add a new page just for completeness. Instead, choose something you're interested in or passionate about.
We do not have to list every ability a class can use, every mob in a dungeon, every NPC in a city. Focus on important or class-defining abilities, key encounters, famous identities. If you want, link to other wikis or resources (including the Curse database, Ten Ton Hammer etc) for more detail.
Be Accurate Edit
Use direct sources, but not vague references to something you heard on a forum somewhere. Don't simply re-word an article found elsewhere -- write from scratch. Consider asking someone from SOE on the development team to comment directly.
Stop laughing! They do, sometimes, and if you're polite and respect the demands on their time you might get some information from the horse's mouth rather than an unattributed forum post.
Provide Context Edit
Describe the game experience. What is it like to play a particular class? What's the effect of mobs that stun in the middle of a Greystone run? How frustrating is the tenth crafting complication with only fifty action points left?
Specific Guidelines Edit
When adding abilities, remember that the numbers you see on your own skills are modified quite drastically by your choice of attributes and equipment. My general rule has been to try to use the numbers given in the ability description by the trainer, before the skill is actually learned. These numbers seem to be a relatively consistent base figure to work with. Icon names are taken from the icon number that appears beside each when they are selected for a macro - a quick and easy way to find out is to right click the skill on your hotbar.