The Vast and
Heroes Community all have a version.
Seeing as how there seems to be a bit of a tradition, looks like I'm in good company creating my own.
Here are my commandments.
THOU SHALT NOT RULES LAWYER. Also includes min-maxing, munchkin-ing, and throwing a tantrum because the GM said no. My game isn't a video game, where he who has the best stats is king. My game is not a miniatures wargame, where he who knows the most obscure rules is king. It's Calvin-ball, with structure. It's about having fun, not "winning". Some people like arguing rules more than telling a story. Go play with them.
[modified after feedback from Reddit, apparently some people really enjoy arguing the rules.]
THOU SHALT NOT STEAL THUNDER. If a thief and a bard are together; the thief gets first shot at picking the lock; the bard gets to try to bamboozle the townspeople first. The mage notices magic before the warrior, but the warrior gets to spot the ambush first. They can still fail at these things... but they get first dibs.
THOU SHALT NOT DIE OF DYSENTERY. The players spent a lot of time on the characters... background info, personality... If they die in game, make it epic. "Blaze of glory, defending the planet" ="good", "infected papercut" ="bad".
THOU SHALT NOT BE A DICK. also known as Wheatons Law. Do not devote time and effort to ruining the fun of the other players.
THOU SHALT FIT THE GENRE. Don't make a swashbuckling pirate when playing in a film noir detective game... of course there are exceptions.
THOU SHALT HAM IT UP. Play the character you made. Have some fun. If, while staying in character, you can make the game master snort diet coke out of his nose, congratulations, you have gotten the entire point.
THOU SHALT FEED AND CODDLE THE GAMEMASTER. I personally go for diet coke and love me some chili.
THOU SHALT NOT GET BOGGED DOWN IN RULES. It's about having fun. Knowing the weight, size, displacement, velocity, inertia, and angle of ascent of a thrown shield is not freakin' fun. When in doubt, roll the dice, point and shout.
THOU SHALT TRY TO BE CONSISTENT. If the character has repeatedly nailed a playing card with a bullet at fifty feet, then yes (sigh) they can aim for the eye slots of the bad guys armor, even if it takes down your badass nemesis early.
THOU SHALT LET THY PLAYERS TRY. Sometimes the most ridiculous attempts lead to fantastic game sessions. Attempting to seduce the stone golem is probably going to end up in failure... but the jokes will go on forever.