Jean Kazez reviews Pinker's book over here on her In Living Color blog. Although she is a lot less taken with The Better Angels of Our Nature than I was, she has produced a useful and fair review - one that you might want to read to balance the more starry-eyed accounts you're getting from me and others.
A couple of quick comments. I think she's right in her comment about the "syllabular" nature of the book. It is not overtly thesis driven, and Pinker does not develop a simple argument for why human beings have, by and large, become less violent and bloodthirsty over the centuries. He identifies factors, certainly, but they are complex, and much of the book simply teaches us the facts from numerous data sets.
Although I agree with this, I don't think it's a great problem. Pinker has given us something that we need - overwhelming evidence that the idea of increasing bloodthirstiness, culminating in a horrifically violent twentieth century, is a myth. He sets the record straight, while not flinching from the horrors of the world wars and the historically recent democides in Germany, Russia, China, and elsewhere. That is, itself, an enormous service, and the book is so well written that it is entertaining in event.
Furthermore, if the reasons just are complicated we can hardly blame the author for not insisting on a simple thesis. It's not as if he has nothing at all to say about the causes of the relative decline of human nastiness; it's just that what he has to say matches up to the messy reality.
Jean does have some convincing points about why the fall-off in cruelty to animals may be exaggerated by Pinker, and why the problem continues to be a serious one. This, of course, raises the question as to whether Pinker may be wrong, or over-embellishing, on other issues. Despite the book's huge size, each topic could be explored in even more depth, and the picture might then change. Clearly enough, Pinker hasn't written the last word on any of these issues.
I'm not so taken with the point that the absolute amount of suffering in, say, a genocidal event increases if the population of societies and the world increases. While that is true, and many of the events of last century were horrible partly because of the sheer size of the populations involved, I don't think it is a criticism of Pinker's main point.
Sure, we ought to oppose acts of genocide, manufactured famines, and all the other monstrous events perpetrated by crazy governments. Yes, the scale of these will be greater, making them even more horrible, in a sense, as relevant populations increase. Nonetheless, consider a very large society where few people, relative to the overall size of the population, settle their differences in violent ways. It's quite meaningful to say that this is a less violent society than a much smaller society in which a large proportion of the population is resorting to violence. That is so even if the large society suffers a greater absolute number of murders than the much smaller one.
In the smaller society the average person is more likely to have a violent disposition. The likelihood that any given person routinely dishes out violence is greater. Your chances of being murdered if you live there are greater. The general ambience that you experience will be more pervaded by cruelty and viciousness. The smaller society is going to be brutal and brutalising. It is the one that urgently needs a better government and gentler mores, etc.
If modern societies are getting more like the large society where most people are peaceful, and less like the smaller one where, let's say, folks resort to killing and injuring each other at the drop of a hat ... well, that is worth knowing, and I think it's perfectly legitimate to call it a decline of violence. It's not misleading. No one is being tricked into thinking that, for example, more people were killed in the Albigensian Crusade (of the order of hundreds of thousands, maybe approaching a million on some counts) than in the Nazi Holocaust (between five and six million Jews murdered).
I'll be reviewing the book myself - for the ABC Religion and Ethics Portal - so stay tuned for that. I understand that Pinker himself will have a piece there fairly soon, so you might like to look out for it.