
One thing I noticed about driving a few years back is that, while the rules are explicitly codified, most people seem to have a favorite few that they cling to at the expense of the rest. The favorite few vary considerably by person, of course - e.g., driving in the leftmost highway lane; following proper turn order at stop signs; correctly interpreting the "go ahead" sign from another driver - which means everyone gets to feel smug and self-righteous at everyone else, but at the price of occasionally being mad as hell at everyone else.
I think the same thing may apply to manners. There are two pieces of etiquette that seem to me not to be widely known, and the violation of each strikes me as a minor but pretty annoying offense.
1) Taking a seat on a crowded bus. One basic principle of etiquette that people seem to fall back on, when they're actually trying to be nice, is "if a desired thing is in limited quantity, leave it for someone else." I want to note that this should not apply to seats on a crowded bus - the kind of crowded bus where half the occupants are already standing. If you are on such a bus, and you are by an empty seat, park your butt in it. I don't care if you're getting off in one stop; I don't care if it's a seat for the elderly (unless, of course, someone elderly is actually present). The reason for this is that the only thing worse than having to stand for a 15-minute bus ride is having to stand squished between two complete strangers for a 15-minute bus ride. If you sit down, it gives us standees all a little more breathing room.
2) "When are you going to have children?" This is never an appropriate question to ask someone unless you are dating that person. Its slightly more palatable cousin, which omits the presumptive "When?", is still not acceptable in most situations. If you do not know a person's reproductive status, this is probably not by accident. There are cases in which people will not be offended at being asked, of course - for instance, good friends of mine are allowed to ask things like this without making me feel violated, whereas random new acquaintances I run into at a party are not - but the safest course of action is just to not ask.
The corollary to my wishing that others would follow these social rules is that there are likely to be other social rules I am either clueless about or unfairly contemptuous of. Anyone who feels memetically inclined, come up with a few of your favorites and post 'em. Maybe eventually we will have a polite society.