In Why Nations Fail, his new book with collaborator James Robinson, he argues that the wealth of a country is most closely correlated with the degree to which the average person shares in the overall growth of its economy.
It's an idea that was first raised by Adam Smith but was then largely ignored for centuries as economics became focused on theoretical models of ideal economies rather than the not-at-all-ideal problems of real nations.
According to Acemoglu's thesis, when a nation's institutions prevent the poor from profiting from their work, no amount of disease eradication, good economic advice, or foreign aid seems to help. Read more in the NYT.