It depends on where you are at, when you are there, and whether or not the people you meet have an attitude problem. Your own behavior will usually determine how you are treated, but not always. People as a whole seem to be a lot more willing to screw over others nowadays, though. My personal observation is that this does not apply equally in all places.
The where: Folks in rural areas often seem to generally be a lot more friendly than those in uban areas, I'm not sure why. People in nearly all the Midwestern US are very friendly, those in Boston and the San Francisco bay area are decidedly unfriendly overall. The demeanor of people in different countries also varies, sometimes for the better, sometimes for the worse. Mostly for the better, I think.
The when: Anyone entering an area when there are unusually high tensions will find it rather unhospitable. People might behave in a much more civil manner at other times. Areas that are safe during the day may become dangerous at night. People all across America were more accomodating and supportive of each other following the 9/11 attacks than at other times. For some reason, this seems to happen a lot during disasters.
The world does seem like it is becoming a more agressive, cutthroat, hostile place. Part of the problem may be that individuals adopt that outlook and behavior as a survival mechanism, although they find it distasteful. They believe there are few who see things the same way, when, in reality, the opposite is true. As far as friendliness being a weakness to be exploited - being friendly and treating others well does not require people to be fools. Fools will be exploited sooner or later no matter what, just as in times past.
Of course, all these statements are based on my point of view. Others may see it quite differently.