The reason it is bunk is because it is applying a bastardized interpretation of QM incorrectly.
If the claim was that positive thinking helps people then fine -- I don't really need that psychological feel good stuff but I understand that it helps some people. That though is not the claim. The Secret claims that by thinking about stuff you can actually change the probability of it happening.
They cloak this claim in science by incorrectly applying the uncertainty principle / indeterminacy principle. Unfortunately they don't understand what they are talking about but it doesn't matter since they know that the fraction of the population who actually understands QM is tiny and that by cloaking it in science language they will gain credibility with people who have never heard of quantum entanglement.