Anyone know a decent website or other resource that can give me a brief discussion (preferably in English) of things that might be considered common knowledge to a Japanese person?

That seems like a very broad topic and I don't think a brief discussion would cover it. (Could you list everything that's common knowledge to people in your country in a brief discussion?)

Could you be a bit more specific about what you're looking for?
This book provides a pretty good summary of Japanese customs from an outsider perspective:
First edition
Second edition