I lived in Brooklyn for about 4 and half months myself (Prospect Place area - lived on Prospect between Vanderbilt/Underhill right on the Q line).
As far as it being 'dangerous', any place can be dangerous. Just be street smart and avoid area's that look bad, because chances are that it is a bad area. Don't ride a subway line too late at night, unless you know the number of people that are on that line, and if you have to take one late, sit near the middle in the car that the conductor is in. Avoid most of the Bronx and don't go to far up into Harlem, it's not all bad, but once you get up to 110th or so, it get a hell of alot uglier. Time's Square can be another ugly place, there's usually safety in numbers, but there are some shady people up there. On 8th between 42nd and 48th there's still quite a few adult bookstores with 'hustlers' outside of them, i.e. asking if you want women or drugs. Don't even say no to these people. Just look down and don't aknowledge them.
As far as the people being rude, I think that's far from the truth. If you think bluntness is rude, then you will never suceed on the east coast. I personally enjoyed it more, when you go to the grocery store it's not some woman talking about her 12 grandchildren while slowly writing a check, you slap your purchase on the counter, get the total, pay, then get the hell out of the line. It's very fast paced, almost too much at times, but there's part's of that I enjoyed.
I myself would probably never move back there (mostly because of a bad relationship) but I think it's a great city.