As a photographer, I have experienced feeling that I have touched people in a ways I wish hadn't. An accidental invasion of privacy, so to speak, which made me feel as if I had commited the rudest form of sexual harassment - without even being aware of it. In photography, One day, you can take a photograph of someone who is not wearing any clothes, but it will be okay. The next day, you can take a picture of someone who is fully dressed, even if you don't see their face, and it is the worst of possibly imaginable sin. What is okay in one situation can be wrong in another.
Legality
Many countries in Europe have added the European Convention on Human Rights as part of their set of laws. This convention has something that is devastating to privacy, called Section 10.
"Everyone has the right to freedom of expression. This right shall include to receive and impart information without interference by public authority and regardless of frontiers." In practice, this is the same as the 1st Amendment in the American Bill of Rights - the freedom to expression.
The conclusion drawn from the 1st amendment and Section 10 is that you can always take pictures. Even on private property, you have the right to photograph anything you can see.
Morality
Despite of something being legal, it doesn't mean that you should, though.
Several minority cultures believe that taking a picture is the same as stealing somebodys soul. Taking a picture of a member of such a culture is inexcusable. Upon having taken the picture - as far as they are concerned - it is too late.
If you, as a photographer - especially as a professional photographer - make the mistake of taking a picture of a member of such a minority group, you have fucked up beyond forgivenness. Call it a breach of professional conduct, or a kick to the shins of common sense.
Other times, however, you meet people of whom you really couldn't have known their aversion to photography. I have experienced taking a picture of a couple looking wonderfully in love. When they realised I took the picture, the male half of the couple came over and asked me for the film. Apparantly he was married, and didn't want me to publish the picture. What was I to do? I decided to promise him to not use the image. I gave him my card, showed him my ID, and left it at that. He is a friend of mine to date.
I haven't been a photographer for all that long. Situations like that will arise again, I am sure. And I am certain that modesty, along with a dose of appropriateness, will get me through those situations.
I have a few friends who work as wartime photographers. In the job, they see some of the most horrible things known to man. The pictures in the dailies are the mild versions of some of the pictures I have seen people come back with. And the pictures never go outside their photo albums. Why? Because some things don't need to be shown. A man far wiser than me said to me once: "These things are not secret, but they are sacred, and should not be taken lightly".
I know of no respected photographer who didn't have respect for the subjects s/he photographs. And - even if you aren't the best photographer in the world - showing respect will get you the respect you need to get a good start in the lion's den that is Photography.
Postscript
This writeup was originally written as an answer to a different node, in which the author writes "But, I get to claim the moral victory. I helped a poor backward immigrate learn one of life's little lessons in this democracy we call Canada. Too bad I didn't get to keep the pictures though." I decided to post it under this node instead, however, as the node does not deserve a reply. I just hope not too many aspiring photographers are that arrogant. Or, more eloquently: Be Appropriate.