He ends up with an account of morality that's simultaneously simpler than many such accounts but that leads to more irreducible difficulties. Simpler because Haidt thinks moral beliefs arise intuitively, with reason invoked later (if at all) to explain why. Further, Haidt's research points to six broad moral themes--care, fairness, liberty, authority, sanctity, and loyalty. All of these have been observed across cultures and throughout time, and all of them are intuitively accessible, although different cultures prioritize them differently.
So in a way, this is a Jiminy Cricket system of ethics: if you go with your gut, you're probably right, even if you couldn't explain why under aggressively contrarian questioning. Some of the funnier passages in the book describe studies Haidt's various teams have done involving disparate subpopulations. When asked whether it's okay to have sex with a dead chicken and then eat it, for example, no one is more likely to reason himself into a grotesque perspective than an earnestly liberal student at an elite American university. Everyone else immediately responds that that would be gross, and gets irritated with the interviewer for asking stupid follow-up questions.
Haidt's account suggests that moral differences may be hard to overcome, though, for the reason mentioned above: even if these big moral themes are commonly held, different groups prioritize them differently. Haidt cites a study in which American and Japanese participants were shown a drawing of a line in a square. Then they were given two cards, with the square reproduced at a different scale and the line removed, and asked to draw a) the same size line that they saw in the first drawing, regardless of the square b) the line relative to the square. The Americans were better at the first task, the Japanese at the second. The theory was that the individualistic Americans always saw the line as a separate entity from the square, whereas the Japanese saw the line and the square as components of one thing.
From that straightforward example you can anticipate more meaningful clashes: between people who prioritize liberty and people who prioritize authority, between fairness and loyalty, etc. We can also anticipate that because all of these themes have intuitive underpinnings and indeterminate value, these clashes may seem intractable. This is where Haidt gets into American politics: if Tom wants lower taxes and Steve wants a bigger safety net, they're both making a moral argument, about liberty and care respectively, and neither value trumps the other.
It's is a challenging read in the sense that Haidt's effectively rehabilitating several moral concepts, like authority and sanctity that Democrats often associate with oppression, and that classical liberals (like me) are more likely to acknowledge than appreciate. It's also a challenging book in the sense that you have to keep stopping to think. And Haidt's kind of explanation is too heady for a lot of what goes on in American politics. But if you're looking for a book to upend assumptions, this is the one.