I enjoyed this book. It's not as deep or intricate as some of Stephenson's books that I've read, like Cryptonomicon* and Anathem. I'd compare it favorably with Michael Crichton's best stuff**.
Possible explanation for its brevity and looseness from his website: "Zodiac: Written over a time span of about a month after the collapse of an earlier project had left me feeling desperate, this developed into an engaging yarn that I still look on with affection." My interpretation: he kind of banged this one out. I get the impression that his normal method is slower.
ST reminds me of my uncle. My uncle is definitely NOT an environmental terrorist, but he is an aging hippie with PhD in plant biology and a deep interest in environmentalism and climate change. He's completely reversed my thinking on climate change and how much the government should be investing into new energy tech (a lot). It did not take a lot to imagine a guy angrier than my uncle deciding to do something more active than going to community meetings and writing to his senators/representative.
*Little shout-out from Stephenson to HP Lovecraft with this title, I suppose.
**My favorite Crichton book that I've read is Prey. It's about nanotechnology and AI based on animal behaviors like herding. With modern CGI, it could make a good movie.