Feynman's secret is that he was a legitimate super-genius with the charisma of a normal guy who happened to luck into all his achievements. In reality it couldn't be farther from the truth, but there's nothing more appealing to an average person than an exceptional person who gives the impression that being like him is achievable.
Feynman was not really framing himself as normal guy who happened to luck into achievements. Nor came accriss as one. His writings are very carefully written and makes you know and feel he is awesome. The way he describes his own dialogs with others also let's everyone know he is super smart and awesome.