I really like this post over at Mystic Chicanery. Every guild, when raiding, has its own unwritten rules of play. You just know that by the end of your raid, the MT will be drunk, the priest will have died about five times, and if you insult the healers, you might or might not get healed. It's these little quirks that make raiding so much fun-- sure, the game is a good time, and downing bosses is definitely an achievement. But it's having fun with the people you're raiding with that make the game worth playing.
So what are your guild's unwritten rules? I find that they usually revolve around people-- someone does something that gets them remembered, and then everyone knows the joke after that. Guildleaders tend to set a lot of the unwritten rules as well, since they're there at most of the raids anyway. And some of the rules just plain have to do with etiquette-- who belongs in group 1, who explains which fights, who runs the tasks of main looter and calls boss phases.
None of this stuff is programmed into the game, obviously, but of all the hundreds (and thousands) of guilds out there, each one, I'd bet, does things their own way.