As in his previous Better Angels of our Nature, he provides ample empirical evidence to support what may seem to be a counterintuitive thesis, namely that seen from a 20,000 foot view, the world is getting better over time, and this is entirely due to embracing reason, science and humanism. Problems remain, but our best hope of solving them is to continue applying these enlightenment values.
It's an ambitious book, and so there are some things that I didn't agree with. I do think that he glossed over the threat of climate change a bit. Though he certainly acknowledges it, he seems to be putting a little too much faith in geoengineering to save the day. He also addresses the question of why it seems that everything is getting worse, not better, but I wasn't persuaded his explanation was sufficient. While generally well measured, he sometimes says provocative things ("Intellectuals who call themselves 'progressive' really hate progress.") only to back off a bit later. Still, on balance, I think Pinker is always worth reading, and this was no exception.