"Where does the term "latchkey kid" come from? Why "latchkey"?"
As Luci45 wrote, a latchkey is just an older term for a house key. The reason why that particular nomenclature was chosen was because the people putting forward the idea of "latchkey kid syndrome" as a descriptive term for kids who developed problems because both parents worked tended to be older, more conservative people. Even my parents, who grew up in the '50s, found the term to be a bit dated.
"Aside from general civility, do we all get along on this forum because we all watch the same TV shows, read the same web sites and comics?"
I suspect it's mostly because people who gravitate toward etiquette discussions tend to be more concerned with civility than the general public. I agree with others that there's a lot more diversity in likes and dislikes than you'd think at first blush, because as the others said, posters tend to self-sort so when you enter a thread about a particular show/book/whatever, you're going to see posts from other fans and those who aren't interested will just skip it.