For people with a conservative bent, a lot of this book will be "didn't everyone know this already"? But for folks of the liberal bent -- like Haidt, although his research for this book migrated him to what he sees as "moderate", it will be something of a struggle.
Sadly, I'm sure that Haidt is due to discover that his observations about human nature may be hyper proven as the liberal establishment punishes him for his heresy of using actual science to point out some fairly obvious things about human nature that would seem to indicate that conservatives are not just "stupid and evil".
First, we are not rational beings, we are RATIONALIZING beings. The book carries on the excellent rider/elephant analogy from "The Happiness Hypothesis" and builds off it. The Rider is best seen as the Press Secretary for the elephant -- the elephant does something or "leans" in some direction and the rider dutifully develops a case for the elephant. Humans developed into "hive creatures" (like bees) that could specialize labor and cooperate without all having to be related. Morality is the "wetware" that we use to create and enforce the rules to do that -- our "rider" (consciousness) was created so that our "elephants" (subconscious) could operate this way.
The Six Moral Senses:
On #3, liberals think of "equality" and completely forget about proportionality -- or Karma. One of the huge problems in cooperation is the "free rider problem". Haidt covers this and why it is impossible to have cooperation without "punishment" (sanctions) against free riders.
Liberals are nearly blind (or claim to be) on 4,5 and 6. It turns out that when tested, the "moral modules" for even Sanctity are there and working in the liberal brain just fine -- they just don't want to admit it because in their view it seems "less enlightened" to admit that degrading things are degrading.
I believe that this book is an EXCELLENT base to at least attempt to open some lines of communication between liberals and conservatives, but I suspect that Haidt is in for a shock -- maybe somewhat equivalent to the shock that Edward O Wilson wrote "Sociobiology" back in the '70s.
The "divine faith" of liberals is that there is no God and man is an infinitely malleable blank slate. While proving that there is no god (or that there is) is not going to happen, it is scientifically known that man is not a blank slate, and at least in the "next few millennia" not likely to be improved upon much. Wilson was trashed for stating the basic outline of what a "human nature" was likely to be, now here comes Haidt with some fairly solid research showing what it actually is.
As Wilson outlined in "Consilience", the more science moves forward, the more we begin to see the fact of an intricate and complex human that is no less difficult to mold to our desires than ecologists are realizing the ecology of the planet is. We are each little ecosystems honed by selection (or created by God) to interact within the the planetary and social constructs that we are born with and into.
Reality has never been very much of interest to the Progressive Project -- now about 100 years in, with all of the progressive nations facing economic demise, even the social sciences start to point out that reality is not in line with the progressive vision. My guess is that the response is not likely to be very reasoned, but rather very emotional.