Because the last one got such a response, here's another Caplan blog post.
As a matter of sincere discourse, I found the most stimulating, interesting, probably-most-true-as-I-see-it critiques are to tenets 1, 2, 5, and 8. The hyperlinks are all dead, but point to corresponding sections of the syllabus for his labor economics class.
Critique: High worker productivity plus competition between employers is the real reason today’s workers have a decent standard of living. In fact, “pro-worker” laws have dire negative side effects for workers, especially unemployment.
Tenet #2: Strict regulation of immigration, especially low-skilled immigration, prevents poverty and inequality.
Critique: Immigration restrictions massively increase the poverty and inequality of the world – and make the average American poorer in the process. Specialization and trade are fountains of wealth, and immigration is just specialization and trade in labor.
[. . .]
Tenet #5: Increasing education levels is good for society.
Critique: Education is mostly signaling; increasing education is a recipe for credential inflation, not prosperity.
[. . .]
Tenet #8: Overpopulation is a terrible social problem.
Critique: The positive externalities of population – especially idea externalities – far outweigh the negative. Reducing population to help the environment is using a sword to kill a mosquito.