Earlier quoted context omitted.
I'd suggest, "The Tough Parts", as you mention in the preface. Not sure who your target market is but as a professional developer who writes javascript daily, "The Tough Parts", piques my curiosity and sounds like an interesting challenge. "You Don't Know JS" evokes a more negative reaction.
fyi: there were some who felt that it would detract from or dillute the brand "the good parts", which is also an oreilly title.
That said, from a marketing perspective, they've got a great unique cover and a solid series title. If more than the original 5 come around in the future, they'll be easy to identify.