What do I mean? Simply put, watch this Dark Souls pvp montage:
Notice anything? If you said that every fight started with some kind of gesture go get yourself a cookie. It is the unspoken, but ever present rule of dueling in Dark Souls that every duel begin with some kind of gesture in greeting. It is also understood no lagstabbing and no Estus (though it never seems to stop people doing it). So I say lets make rules of engagement for other games with multiplayer, lets hear what you come up with.
P.S. If you invade my game in Dark Souls and find yourself in a three on one, you have no right to complain about being ganked because I didn't summon you into it, you invaded me yourself with a Red-Eye item.