I don't define common sense as someone who doesn't have the same knowledge or beliefs as I do.
To me, someone who is book smart, but has no common sense, would be someone who graduated high school with honors, but can't "get" a simple pun or joke. Someone who has a masters degree, but basic things just "escape" them-like following the directions to assemble a baby toy.
My brother, for instance, is working on his masters degree-but has to have my parents bail him out all the time because he can't handle basic life skills like driving without getting into a car accident, keeping a job, or paying his bills.