Is the law of attraction a spiritual placebo?

Yes, it is "a spiritual placebo". There is no "secret". It's just a repackaged version of Gnosticism. The real "secret" (if you want to call it that) is quite simple really. It's called the Golden Rule. Do unto others as you would have done unto you. Treat others the way you want to be treated. It doesn't get any simpler than that.