How about this:
No jobs. You heard me. This limits people to only roleplaying the way the game is letting them. RPs should focus a lot on the characters themselves. It’s better for the people themselves to decide who and what their characters are.
Find a theme or story that encourages fun passive roleplay. When people don’t have anything to do actively, a good themed RP encourages conversation. For example, if your RP is in a, say, a post-apocalyptic setting, players will want to talk to others because they have a slight sense of role (AKA they get to talk about the goings-on in their world instead of “give me guns”.)
Make it so that you can whisper, shout, etc. And make sure you have seperate IC and OOC chats. No one likes to roleplay in the chat box when they see other people having OOC conversations. Also, make sure the people who can see/hear a person talking in IC chat must be at certain distance. This can somewhat prevent metagaming over the IC channel.
Make weapons hard to obtain, for the Maker’s sake. No one likes seeing an 8-year old kid getting a gun and shooting up the place just because he bought it for $100 dollars. Eventually this will weed out the common dummy from your server.
Basically, take precautions so the player base can’t run rampant and/or stupid. The player base is the absolute most important factor in a quality RP server. You must encourage good roleplay and “make-believe” in the server.
Inspired by NecropolisRP because it’s the only server out there right now that seems to be doing it right. The GMod community deserves better RP.