For races/social norms: include anything that's different from the standard. For races, this generally means anything that doesn't fit the Peter Jackson mold; I'd call it "the Tolkien mold," but it's 2016, so I think we all know the score. For social norms, assume the characters approach the world the same way the players approach theirs: while a Middle Ages scholar (or a high fantasy buff) might take it for granted that, in a Middle Ages-based setting, the local high priest is an individual of staggering political power and reverence, the players aren't likely to think of him as being any different than the minister of the Methodist church 'round the corner.

If you're changing to a new setting with players who are quite experienced in another setting, it'll probably be more helpful to point out how this setting is different from that one rather than from any tabula quasi rasa assumptions.

Salient point: don't talk about how things are, talk about how things are different.