Bostonian here.
My work has taken me to cities all over the country, and I have to say that I've found New Yorkers to be the most considerate and helpful big city denizens in the US. The picture of the typical New Yorker as an obnoxious ogre is a phony movie and television trope.
People mistake adaptations to the pace and concentration of urban life as unfriendliness. Yes, people don't smile and nod at everyone they meet as they stroll the length of 5th Avenue, because after three or four blocks they'd need a chiropractor. But approach one of those people on 5th Avenue for directions, and most of the time he'll be pleasant and eager to be helpful.
Of course, you take your chances approaching strangers in any big city, but I also think that a lot of the treatment you receive is determined by the attitude you bring with you. I've heard wildly different reports on the infamous rudeness of Parisians, but the reports are usually a reflection of the kind of person making the report. Courteous people tend to be met with courtesy wherever they go, and obnoxious people get a rude reception.