Food culture signifies something about society, so what is American food, and what does it say about us?
American food is soul food. Examples of soul food is barbeque, hamburgers, ice cream. french fries. grilled cheese, mac and cheese. biscuits, fried chicken, fruit pies, steak, etc. Soul food is marked by a lack of refinement. a lack of sophisticated preparation (e.g. turn and burn), and few (typically less than five) ingredients per dish.
Why would America not only espouse this unrefined food, but in fact celebrate it, to the point that you can find very refined versions of soul food in well-regarded, expensive restaurants. It must have something to with who we are.
Not to get all patriotic about it, but who we are is a state born of a revolutionary spirit, a young state, a state in adolescence that managed to adopt the stature of the unipolar for a time by dint of industry, ingenuity, war, and market control. To power a revolution, you better get it in your soul. A state in flux has no time or history for refinement - instead go for the calories (cf. American obesity epidemic). Hence, the flavor profile of soul food is typically salty, fatty, and sweet.
While American is not an ethnicity, America is filled with fantastic ethnic food. Perhaps the reason why Mitchell Davis' guests have such a hard time defining American food is that the adolescent food culture, like the adolescent, is trying desperately to both define itself and resist definition. Never comfortable with themselves, the adolescent actually puts value on their undefinability and their refusal to be pinned down.