I find that one of the causes for wildly different levels of code review (and value derived from them) is a lack of training. There is a real lack of materials

I find that one of the causes for wildly different levels of code review (and value derived from them) is a lack of training. There is a real lack of materials for explaining how to do a code review, how to do deal with the human aspect of giving feedback, what is/is not valuable to talk about (arguing over tabs vs spaces should not happen in a code review). Most of my experiences have involved a trial-by-fire process - new engineers receive a few code reviews from more experienced people and that is your "training".