Our first two -
No smokers (one player has sever asthma and smoke hanging on clothes, even if 2 or 3 hours old, in a room for even a short length, can cause a major attack).
No alcohol, and don't show up inebriated or tipsy.
Beyond that -
All games are PG-13, respect the boundaries.
Be respectful of other players and the GM (i.e. don't be a jerk).
Minimize swearing, even in character (See PG 13 above).
Know the basics rules of the game being played.
These pretty much are group consensus - and this group has been together for 20+ years. It works for us.