1. There is a difference between IC and OOC
IC stands for In Character, a mode that requires all players to play their charaters role. Call it roleplaying in progress. Or lights, camera, action. This is when you will find people in their roleplaying personas, acting and talking like how their character is suppose to. OOC means the opposite, Out Of Character, where you can all relax, be yourself and talk about what's on TV tonight. In chatrooms, OOC talk is required to be enclosed in brackets, (()) or [[]]. Some has a separate channel for OOC, which is much better then peppering the roleplay with OOC comments. Similarly, OOC should not be taken into IC. Say for example two people who are friends IC and OOC. One day, these two friends have a fight. When they meet IC, their characters start fighting for no IC reason.
2. You don't know me until someone introduces us
The way the Internet is designed requires us to have a name to identify us by. Channels and chatrooms display the names of the users, but the problem with that is that some people think roleplay channels works along the same lines as #chatzone. Although its perfectly fine to say "Hi Jacob" to a chatter named Jacob who just entered a general chatroom, it is a different thing all together in a roleplaying environment. Rule of the thumb, if we have never met, you don't know me, I don't know you. Referring to a stranger by name in an action (e.g. Alice smiles at Jacob) is fine. Calling them by name is a roleplaying boo-boo.
3. Actions are in 3rd person.
Some perfectly good sentences get spoilt by confusing the 3rd person and the 1st person (narrative). Things like "Jacob slips down the path quietly, my hand on the hilt of my sword." Or "Alice walks over to Jacob and puts my hand on your shoulder." Roleplaying actions are always in the 3rd person. "Jacob slips down the path quietly, his hand on the hilt of his sword." "Alice walks over to Jacob and puts her hand on his shoulder." Also, remember that actions are in present tense or present continuous tense.
4. Be Courteous
Roleplayers are people too. It's a nice gesture to say "Thank you for the RP" or "It's been nice RPing with you" when you're about to exit a session. Or asking if you may join in on a session that seems to be already in full swing.