It really depends on what city you live in or are visiting, and of
course, every individual reacts differently to having their pic
taken, too.
Here in Atlanta people are generally much less friendly to having a
camera aimed at them. If you are around the big touristy areas
such as the Aquarium, Zoo, or Centennial Park, then you have a lot
of folks from all over the world and you can get by with it. But
if you go into the heart of downtown, you're mixing it up with
local drug dealers scattered around, as well as more rural native
southerners, all of whom have this "big brother watching me" hangup
thing going on - you're likely to catch an image of a fist about to
hit your lens (or bullet) than a face. LOL. So the rule here in
most areas is, "ask first".
I pretty much agree with your assesment, relative to the ease of street photography in NYC and other large cities. I do however think you overstate the dangers of shooting in downtown Atlanta a bit. Like anywhere, you have to know what areas are safe and which areas aren't. In downtown ATL the Fairly-Poplar district and Peachtree Street corridor are fine. The area right around the Five Points Marta Station can be dicey at times, but usually okay. None of it is particularly safe at night, becuase the streets empty out at 5:00pm. This will improve as more and more people move to the downtown area, though.
I do think it is a good idea to get to know the locals in the areas you plan to mine regularly for photo ops. It's one thing to stroll down 5th Avenue in NYC snapping away, and another thing entirely when you are shooting in a neighborhood area in Atlanta (or any other town for that matter). You need to put the locals at ease and gain their trust if you don't want to be hassled all the time.
I shoot regularly at the commercial center of my neighborhood, Little Five Points. It's a funky gathering place for all manner of funky people, most of who are in their 20s or early 30s. I am over 50, so I tend to stick out in that crowd. But I have been shooting there for years, and the regulars know I am trying to document the unique character of the place so they leave me alone. The youngsters who drive in from the suburbs on the weekend are more likely to give me a hard time, but I just tell them I have lived there for over 20 years and they are in
my house; not the other way around! But its all good. I chat them up, show them some photos, and go on my way.
So my suggestion is to own up to what you are doing. Lose the long lenses and get in close. Be real clear in your own mind why you are taking pictures in a particular area, and communicate that to the people you will see regularly. What people don't want is to be treated like some exhibit in a zoo ("hey Ma, there's one of them spikey haired slackers! Quick, get a picure!"). Let the locals know that is not your intent, and everything else will be much easier (you won't need to ask permission for most shots).
--
Bob