The book devolves into policy opinions that have been absolutely torn apart, but the parts about inequality and inheritance shattered some long standing assumptions.
For example, it points out how generational wealth being gone in 3 generations is not for the reasons people extrapolated, the common assumption being that the person that earned it had a lot of discipline, while the subsequent generations experienced complacency and excess, as that was just anecdotes with no data. It replaces them with data that highlights population growth alone influencing this outcome:
In periods of large population growth inheritances were simply diluted to the point of having little efficacy for heirs. In America the free population was 3,100,000 in 1790, while 308,000,000 in 2010. The last census before the book came out. In comparison, France in the old world had 30,000,000 in 1790 and 60,000,000 in 2010. Old world wealth has tended to stay in the same families for centuries. The US is experiencing the same thing amongst some families and as more families get better at estate structures that work for them, a lower birthrate and age of the country, but all of it challenges the common assumption and point of generational wealth.
There are more illuminations around the movement of capital in that book.