On the legal issue, in most civilised places around the world, you can take a photograph of someone in a public place, publish the photo and sell that photograph without the consent of the people in the photo.
However, if you take a photograph for a commercial purpose (which generally means using the photograph to market or advertise something other the photograph itself) and you don't have consent (ideally a written model release form), you could be liable for damages. The damages would generally be the financial loss suffered by the subject. This could be the income you earned from the photograph. It could be a reasonable modelling fee. You would be liable for more damages if the use of the photograph led to emotional distress, injured their reputation etc. For example, you used the photograph to advertise your local adult shop.
No sane person would start a legal action if there was no prospect of reasonable damages (unless you really annoyed them). Therefore, you just have to be worried about the defamation side. The good news is that you control this risk by deciding how to use the photo.
FYI, stock photography sites will require you to hold model release forms.
Also, there are a lot of landmarks around the world that will require you to have authorisation if you want to take photos for commercial use. Otherwise you might face copyright problems.
Therefore, from a legal perspective, generally if you do it for fun, or to sell the photographs you take, there's no real problems. If you have identifiable landmarks, or use the photos in advertising or promotion, you should have approval.
Different people have different views on the ethical issues. I always have reservations about uploading photos, but I'm not really into street photography and I'm very privacy conscious. Whereas my sister uploads page after page of random people. She's probably got half of Australia on there.