Guild Chat should always be considered In Character, for idle chatter and non-character related conversation, please make sure you join the 'sotmooc' channel.
Don't metagame, many other players don't like it when your character suddenly knows things that should ideally be outside of their realm of knowledge. This includes, but is not limited to, foreshadowing future events that your retail character has experienced. Please play in the here and now, and not include events that have yet to happen.
Don't assume another player's actions/reactions. Role play is a two-way conversation. Try not to assume or act out how others will react to your characters words or actions. State your action and wait for the others to respond.
Remember your turn. This is a hard one to keep in mind, particularly when you are excited... Keep in mind the time it may need for others to respond to your character. If you are in a group, wait for those around you to respond to your actions or words. This doesn't have to include the entire zone, just the people surrounding you.