I grew up on Long Island, in NY, and I live in Boston now, and I've traveled a whole bunch.
When I'm in Boston, I miss a lot of food from NY. Real NY bagels are the first to come to mind. Good Jewish deli (hot pistrami on rye with a broccoli round knish--they don't know from that in Boston). And genuine, Italian pizza shops. And quality Chinese food buffets. There were so many wonderful ones on Long Island.
When I'm in NY, I miss ice cream. Boston and New England have tons of family-owned ice cream stands with handmade ice cream. I love that every town pretty much has their own ice cream stand, and that they're all so different.
From when I went to Israel, I miss strawberry banana nectars that came in a can that I could get in vending machines. Oh, and Elite candy. Their taffy was incredible. Ooooh. And white chocolate Magnum bars. I still dream about those sometimes.
From The Netherlands, where I lived for four months, I miss readily available Quorn (it's harder to find in the US--hasn't really become popular here yet).