His most interesting comment is that Norse social mores contributed to the collapse - despite having the good example of the Inuit, the Norse either refused or were unable to make certain adaptations to their lifestyles that would have allowed them to survive. As Diamond says: (p. 239-240)
Over the centuries, there was little change in the styles of tools and carvings. Fishing was abandoned in the earliest years of the colony, and Greenlanders did not reconsider that decision during the four-and-a-half centuries of their society's existence. They did not learn from the Inuit how to hunt ringed seals or whales, even though that meant not eating locally common foods, and starving as a result. [emphasis mine]Of course, the Greenlanders had to know, at least to a certain extent, that they were doomed. While these things can of course come suddenly (that's kind of the point of Diamond's book, after all) there are also warning signs, for those who are looking. The point is to recognize the impending change, and to accept it.
Or, you can simply say "Our way of life is non-negotiable", like a certain vice-president. Guess which choice will lead to success, in the end?
(On further Googling, it looks like this quote is variously attributed to Cheney, Bush Sr., and Bush Jr. This makes me seriously doubt that it was, in fact, ever said by any of these men. On the other hand, these aren't deep thinkers, so maybe they're all reading off the same cards. In any case, I think we can all agree that the sentiment is well represented by their actions.)
He also has an interesting account of societies that have actually managed to survive (in some cases, for literally thousands of years) despite having climates that are, in their own way, as hostile as Greenland. So there's always cause for hope!
With that, I'm going to read his account of the Rwandan genocide... and hide the sharp instruments in my apartment, probably.