"Whenever the locals rub blue mud in their navels, I rub blue mud in mine just as solemnly."
- Lazarus Long
From the 10,000 word post:
"Whenever possible, principle behaviours of the non-players in your setting should not be required of the players - if your NPC's rub blue mud into their bellies every day, the fact that your players choose not to do so should not automatically cripple their freedom to play as they wish. Have a setting that does not depend upon a character's behavior matching that of the natives ... this sort of constraint will build up resentment that will ultimately erode the suspension of disbelief, in that it will be perceived that the DM is fucking with the players for personal reasons."
Let me just say ... I love Heinlein, and I love his character Lazarus Long. And I agree absolutely the sentiment expressed by the author in the book quoted above. It's only that I believe in it where it applies to real life.
D&D is different, if for no other reason than that it is often impossible to convey true, actual danger to a party which is often as full of bravado as an old Tom turkey strutting about a week before Thanksgiving. Nine times out of ten, if you have your NPCs rub blue mud, your players just won't get that failing to do so will result in their deaths sometime in the next five minutes.
Some can argue that this is justification for ludiocide - or killing players, if you don't speak Latin. I argue that players are made far too complacent by living in a society where remarkably few are ever killed for failing to obey social standards. Once, you could count on getting socked around by a local mob for failing to tip your hat to a woman. Those days are - thankfully - long gone.
Players are not institutionalized to the idea of cultural misbehaviour having tangible, uncompromising consequences - and if you as the DM insist on punishing them for not aptly interpreting your few sentences to the contrary, you're just a smug little bastard. Maybe, if you spent two or three hours with a white board and graphs, and possibly with a few guest speakers besides, you MIGHT get across to them that the locals will be watching every action they make with the intention of vivisecting their hearts from their chests if that is considered necessary, but I doubt it. The Tom turkey just doesn't believe in Thanksgiving, and arguments to the contrary are not going to make an impression.
When you sit down as DM and devise your setting, keep in mind that your players are incapable of living in any time except the one they're actually living in. Your world, at best, is a Disneyland attraction - its there for playing up the visuals, the occasional feel, even the tactile sensation of the fantasy medieval environment ... but don't kid yourself that the players strutting around said environment are living in any period but the present. They are interlopers, tourists and noobs. If a 40-year veteran of roleplaying found himself in an actual medieval setting, he or she wouldn't last a week. Apart from having to vomit from the smell for the first week, they'd sure enough offend someone by insisting on some dumb ass privilege to speak that did not exist ... and in turn they'd get clubbed to death by a bunch of pastoralists.
I'm not saying this whole difficulty is a bad thing. We are enlightened in this century, and good on us. We make inroads daily towards a greater enlightenment (everywhere, that is, except certain crimson or mauve provinces of certain subtropical regions), and this is a good thing. There's nothing wrong with presenting the D&D setting with the flexibility of the present day. Your players - the fanatic ones aside - aren't looking for the genuine experience ... they are quite happy tackling the everyday problems of hitting with a broadsword and doing enough damage to kill. They don't need to be heaped with social degredation besides.
Now, the DM can certain wallow in such as much as the DM desires. There's nothing that says the NPCs can't cheerfully murder one another over a split infinitive or fashion faux pas. Remember, however, that in your world, most of these culturally established rights and wrongs are in your head! They're not inbred into your players, and yes - if you get dictatorial about seeing them met to your standards - you will be fucking around with the players for personal reasons.
That isn't the game. Too much cultural crap in the setting is like grit in the wheels - it will kill momentum, it will grind the campaign to a crawl and it won't add one thing to the player's enjoyment of the game.
So here's what you do. Have the cultural norms that please you. Have the NPCs play up the cultural norms as far as you, the DM, would like. When the players speak, translate their speech into the speech of the natives, and act as though every word the players say - or every action the players make - is in tune with the local scenery.
If you're very good at your world, and you are consistent, with time your players will fall into speaking back to you the way your NPCs speak to them. It will come naturally. The Tom will walk right into the axe when you present it.
But if you try to get behind and push the Tom, you're in for a rough time.