Historian Timothy Snyder published a critical response to the main arguments put forth by Pinker in this piece for Foreign Affairs magazine. His criticism essentially boils down to three points.
1) Turf defense: Pinker is a psychologist and not an historian. Consequently his view of history, as presented in the book, is simplistic. Moreover, he uncritically accepts somewhat questionable information about past societies and his data are therefore problematic. I should point out that, even though he rightly considers Pinker's methodology to be flawed, Snyder does think that his general conclusions regarding the decline of violence are correct.
2) Ideological conflict: Pinker, a mild libertarian, is in Snyder's view unreasonably wary of the role of the state. Snyder, a confirmed statist, points out that while Pinker rightly credits the ancient state with lowering the general level of violence, he ignores all the good things that the modern state does for its citizens. Among these are mass education and public health programs that produce the benefits of better lives and a general civilizing effect. The state is therefore to be credited for training people into non-violent patterns of thought that Pinker naively credits to enlightenment thinkers.
3) This brings us to Snyder's third criticism -- that Pinker has a simplistic understanding of historical processes. He rightly notes that Pinker applies a long discredited approach to intellectual history, one that assumes that ideas trickle down naturally from the great thinkers who formulate them and ignores the mediating and often contradictory effects of the institutions and mechanisms by which ideas are spread throughout society. Moreover Pinker, by emphasizing the world of ideas, neglects to note the materialistic explanations for historical change. Snyder then advances what to my mind is an even more simplistic Marxist/Malthusuan/materialist position -- that conflict can be attributed to the scarcity of resources and that the decline in violence noted by Pinker results from technological changes that have in our time produced a temporary abundance of resources.
This is a fascinating and important debate. I expect to be following its twists and turns for some time to come, and of course will be blogging about it.