I know that people are going to think you're exaggerating about Boston, but that happens to be true, especially in the near suburbs around the city. My wife (who's from the Midwest) and I still joke about it from time to time, although we moved 60 miles north from Boston into NH (where people are a bit warmer - though not much by Midwest or Southern standards) many years ago.
In terms of general surliness, my time living in Boston taught me that it varied greatly between groups and neighborhoods. Because of the large number of colleges in the area, people around Kenmore Square (for example) tend to be friendlier (and frequently are non-natives). Beacon Hill types (Jawwn Kerry's crowd) are not often abrasive, but can be distant and stand-offish. The adjacent North End still has a lot of very down-to-earth and friendly older people (Italian, largely). The South End is Gay Central - not mean, but a bit creepy. Much of Cambridge is like the Third World; dirty, smelly and full of commies.
Southie and Charlestown are more like what you describe: insular, highly suspicious of outsiders, with the working-class Irish predominant. The funny thing is, once you are "accepted" by one or more of them, it's almost as if they've vouched for you ("Ahhh, I checked'm out, Tawmmy, an' he's okay."). Only then you can walk into a bar and not draw hostile glares. And that I did frequently as a young man.
I love the irony of it being warmer in New Hampshire.
There basically are three groups of people in Boston:
1. The “Boston Brahmin.” It’s impossible for most Americans to understand how classist Boston is. The name “Brahmin” is a reference to pre-Gandhi India, the only other society so hatefully classist. And that’s what BOSTONIANS call them. Think John Kerry and William Weld.
2. The Brahmin bitchy wanna-bes. By night, Sex, alcohol, black clothes. By day, type-A bitches that would shock Washingtonians. Madly socially liberal, but just to fit in. In reality, they’re profoundly racist. (I live, now, in Washington. Think Nancy Kerrigan’s somewhat unfair charicature.
3. Townies. Resentful of out-of-towners; they’re racist, which is misdirected anger, but they have reason to be angry. (Not at blacks, but at the Brahmin who used blacks to boot them out of their homes to bust up their communities. Certain neighborhoods in Charlestown were 100% white in 1970 and 100% black in 1990.)
4. Assorted outsiders: Blacks, Indians, new Irish immigrants, stranded Yankees fans. Hated by all sides, the’re actually quite friendly.