Hello, I have an old Boston roommate coming to visit me all the way from London next week. I'm a little new here myself, and while I don't want to duplicate her Boston experience, I know what kind of places she likes based on these:
The Franklin (South End): darkish, crowded but not too-too hip, delicious contemp. Amer. food, mostly under $16 a plate
Toad (Porter Sq.): small, also darkish, good beers and friendly, laidback staff, free music of varying quality (usu. bluegrass)
Shays (Harvard Sq.): divey, punky but friendly wine bar
I guess basically anything that feels cave-like, with good wine, pretty lamps and friendly people.