For the sake of defining it, one might say Street Photography is a still documentary of our times - life as we know it or see it. For many it is a mirror to the society, while for some it is a reflection of the photographer’s thoughts, portrayed through the events of real life. However there is no fixed definition. It is open to interpretation, open to experimentation. In the course of time, I have been asked a number of times about the ethics of it all. Street photography is invasive, doesn't matter if you took the shot from across the river or sitting next to your subject. You did capture a private moment. Its voyeuristic some may argue.
The question arises, where do you draw the line? When do you say, that could be a great shot but am not going to capture it. I do follow a few things. For lack of a better term, I am settling for etiquette. The basic principle that I follow is to avoid face shots as much as I can. That does take away a lot of the personal/intimate touch from the shots, but it allows me to focus on other things. Also, do remember that this is street photography and not photojournalism. There is a difference. And that difference means certain things are not acceptable to the SP community. The thing you'll hear everywhere is:
"If people don't want to be photographed, you do not photograph them."
Smile - It is creepy to have someone take your photo and then look all grumpy. A bit demoralising as a friend once said. A smile can make a lot of things work fine. You may end up with an interesting conversation, perhaps a better story than the one you had in mind. A smiling face is more reassuring than a grumpy, scrunched up one.
Thank - It is also important to thank the person for that shot. This does not imply that you walk across the courtyard to say those words. Nod your head, smile, say it (if not then mouth the words) or a wave of hand. The whole point is while the whole act maybe invasive you could still be polite.
Apologise - If someone did not like being photographed and bothered to tell you so, apologise and delete the shot (its digital for most, so should not be a trouble). The alternative is to allay their fears, share your details, strike up a conversation, promise to send them the shot (and do send it).
Remember when photographing the street you are part of the street, the "fly on the wall", "unobserved observer". It is all about people - be nice to them, respect them, and love them. You are one of them.
Finally, to the ones who do not indulge in this hobby, if you do not appreciate being photographed let the chap know - hand signal, words, anything will do. Ignoring will not convey a "No". When you approach someone who just took your shot, do not charge at them. They have probably committed no crime and thus not at all bound by any laws to pay attention to you for your rudeness. Lastly, others need not concur upon your conditions of acceptability. Remember that.
I am no expert on the subject. I am still learning how to navigate myself here. However, it is something I feel strongly about. The thrill of standing on a street corner in freezing cold to end up with that one shot which makes up for all the discomfort, is unbeatable.
Note: The views mentioned here aren't conclusive or definitive regarding the topic. These are my thoughts, my personal guidelines. Street photography throws up many challenges in terms social acceptability, legalities, morals and ethics. The boundaries are never set.