Deadspin recently fielded the question, "What If Baseball's Unwritten Rules Applied to Real Life?" Ever since, I've been considering this question with regards to Magic. We obviously have a complex comprehensive rulebook, but the rules of the game aren't what unwritten rules are all about. Like in baseball, unwritten rules dictate what is and is not acceptable social behavior during games of Magic. I've compiled an initial list of these rules below, but I encourage you to add more in the comments.
In a just MTG society, any player found taking one of these actions would be subject to having one of his rares defaced with a Sharpie:
- Trade sharking little kids or new players.
- Making sexist remarks to female players.
- Slowly pile shuffling in between mulligans with an already low round clock.
- Bringing so much swag to tournaments that neighbors are crowded (assuming the TO is providing enough space).
- Violently riffle shuffling an opponent's deck.
- Announcing or showing an awesome pick during a draft with people you don't know.
- Slow playing even after being alerted to it by an opponent, judge, or other interested party.
- Saying "good game" as the winner, especially if it wasn't a good game.
- Not taking care of borrowed cards as if they are your own.
- Nagging players to trade cards or items they don't want to trade.
- Acting as if you deserve a win because you're "a better player."
- Pressuring someone to concede or split after they've already turned you down.
- Outright dismissing a card, article, or something else related to MTG as having no value, even as others are stating they receive value from it.
- Anthropomorphisizing your deck as an entity that is opposed to you winning.
- Telling an opponent their win was due solely to luck.
- Touching someone else's cards with visibly dirty hands.
- Assuming a female player is there because of her boyfriend.
This is hardly an exhaustive list, so like I said, feel free to add your own additions to the unwritten rules of Magic. Really, though, each of these boils down to one simple adage: don't be a dick.