I think people can be fooled by surface things, Dennett. New York is crowded, for example, so it doesn’t make much sense to to wave at everyone you see. In the country village where I live now, it’s customary to wave politely to people when they drive by in a car. It was like that when I lived down in northern Alabama too.
In NYC, that would be impractical. Your hand would fall off. ;-)
People are also less apt to make routine eye contact, I think, for much the same reason. There are just so many people everyone that connecting with everyone who walked by, no matter how briefly, would be exhausting.
What I notice instead, is that people in Manhattan reserve their connections. They won’t say hi to every passing stranger, but they’ll be very likely to offer directions and advice to a tourist who looks lost or in distress.
If you sit down on a bench in a park with a New Yorker and start a conversation, you’ll probably get your ear talked off. People wait for those “appropriate” times and places.
New Yorkers learn to give each other a semblance of privacy in public spaces, but also how to send and receive signals that it’s OK to break out of that privacy.
I think a lot of the “New Yorkers are unfriendly” meme comes from people who live in less crowded places and who have learned different kinds of social signals.