Thanks for the comments, everyone.
craeonics >> Though #12 will probably only happen to Americans, don't you think?
No, several times, it was some other who turned around and condemn the entire community as "You arrogant Americans." I am not American, but I too use to have the tendency to assume that others are.
As for looking foolish, there's just always a chance of that. No 3 comes from that. You are likely to get a nasty comment because someone thinks you are typing something stupid. The universally least tolerated trait is stupidity or percieved stupidity.
WOM:>> But as I've seen since, the kids (not all) talk to their parents like they are talking to some idiot friends. So I guess this is outdated.
Yeah, that wouldn't work for bratty kids who have no respect for their parents. And it's not always the kids who are acting childish and being rude anyways..