Whenever I put my elbows on the dining table now I do so with a silent apology to my elders and betters. It's hard to ignore old rules, and while no disapproving grandmother ever leaned over with a fork to lever my elbow off the table, that's how many children learned. Elbows on the table, along with talking with your mouth full and eating peas off a knife, was a breach of good table manners, and good manners mattered. Once upon a time parents only needed to say "manners!" with a warning frown to bring a child into line.
Some of the old rules seem absurd today, and there is sometimes a clash between generations. So you get people yelling a lot at the television during "Downton Abbey" or "Pride and Prejudice" because they know that in those days they didn't do that, whatever that was. For example young women, unless very grand indeed, would not remain seated if an older woman came into the room. Mr Darcy, however displeased, would never leave Elizabeth Bennett in the middle of the dance floor when the band stopped playing, he would escort her back to her family.
Not so long ago Noël Coward, writing to T. E. Lawrence who was hiding in the Royal Air Force as 338171 Aircraftman Ross, began his letter with "Dear 338171 (may I call you 338?)". This was of course a joke, but underlying that was a principle. Coward understood the etiquette around the delicate process of advancing an acquaintanceship to something closer, to what was called first name terms.
That concept has now lost all meaning because everybody is on first name terms with everybody else, even strangers. I get mail with "Hi [first name]!" from a number of sources simply because I have been required to put all my names into their little boxes and they, by default, assume that the first name is the right name and that it is a friendly way to address someone. I never use my first name, don't like it, and have often not answered to it by mistake. It makes me cringe, especially when people telephone and say "is that [first name]?". Obviously they don't know me or anything about me, and it's hard not to slam the phone down before finding out what they want.
I don't think that it is a friendly practice because it debases the word. A friend is someone you have come to know, trust and like. Using a first name implies that we have progressed from stranger to friend, and to assume otherwise is not good manners. P. J. O'Rourke said that good manners were a combination of intelligence, education, taste and style. I would add sensitivity.