Well I never did get around to making the huge post I had stewing in the back of my brain, work has just been wearing me down too much.
But I will say that if I had one of my characters running a town or pushing a new culture, there are a couple of things I'd make sure of, even if they're from more of a meta standpoint.
1) Make it difficult for an active character of any kind to be killed, unless their player is obviously trying to force the situation. Even in a harsh society, even for a criminal, there should be enough wiggle room to let them continue playing the character as long as they're willing to, even if it's as a prisoner or a slave. There's plenty of options now for non-lethally dealing with problems, and murder-happy guards killing people out of hand just kills potential RP.
2) Make sure there's fun to be had for characters at every level of society, and put some effort into facilitating it yourself. Don't be that leader that just stands around holding keys. Say you're running a feudal system or a caste system...organize costume parties and tournaments for the upper class, encourage them all to get pet hawks or attend poetry readings and so on. But don't leave the lowly gatherer class out...maybe they're only allowed to wear hemp clothes, but they're also the only ones allowed to swear or spar in public or drink beer or enter mud huts. Give them sickles or pickaxes, load them up with beer, and then as long as no one directly insults a noble and, say, the mining class as a whole turns over a certain amount of resources every year, treat anything they might get up to as beneath your notice. Let them enforce rules and make work schedules and have clandestine meetings among themselves, if that's what they want.
Which leads me to: 3) Give people room to
breathe. I can never shake the mental image of towns as these little squares where the average citizen just sort of stands around under constant scrutiny, hemmed in on all sides by buildings they're not allowed to enter. At all times everyone can hear every word you say and is hyper aware of every single thing you do. Let people feel free to explore, let them hang out in their apartments or in a quiet workshop with a friend...interesting things might happen. (And not
just the sexy kind of 'interesting things', okay.

)
4.) Make interesting rules. The kind that are easy to break, accidentally or otherwise. The same boring four laws in every town has been done to death. I always wish there were more creative ones with lots of misdemeanors mixed it. Everyone working for the town must have a neat and clean appearance. Men must pay a tax if they're not clean-shaven. The farmer who lets the pigs go hungry gets thrown in the mudhole, the couple that forgot to put their clothes back on before coming outside has to pay a fine, and so on. IRL regular people break the law pretty often and it's not a big deal, that's because we have more laws than 'no assault, no theft, don't trespass, don't hunt endangered species'.
5) Perhaps the most important one: Entice those newspawns. A town with a lot of strange customs or laws is going to take a little warming up to, especially if it clashes with the idea the player already had for that character. I always thought I'd like to have people in town assigned as mentors whose job it was to pay special attention to newspawns, take them aside to ask or answer questions and help them settle into a useful role. Leaving town in the first ten days might be...disallowed, for...their own protection. Also in an extreme case there could always be the...Special Workshop for Recalcitrant Young People, located somewhere nice and private, for the ones that absolutely refuse to let go of their generic Cantrian cultural bias...
