The major lesson of Collapse as a whole is mirrored in the lesson that Diamond draws from the respective fates of the Gardar farm and the Huls farm. The basic idea, to Diamond, is that the society that was home to the Gardar farm made choices that harmed its environment in important ways. By contrast, the society that is home to the Huls farm has not (or at least not yet) made choices that have been that destructive.
According to Diamond, the Norse who went to live in Greenland made a number of fundamental mistakes. Those mistakes led to environmental degradation. The most important of these mistakes was to cut down too many trees. By cutting down the trees, the Norse deprived themselves of the ability to make things out of large pieces of wood. More importantly, they deprived themselves of the ability to build as many fires as they needed. This was particularly important because the Norse relied heavily on dairies and dairies need hot water to wash their equipment and keep their food safe. The Norse also made the mistake of relying heavily on turf-cutting to build houses. This reduced the amount of land that could be used (since the turf grew back very slowly) to grow hay to feed their animals.
In these ways, the society in which the Gardar farm existed made choices that devastated their environment, Diamond says. Their choices made it impossible for their society to continue to prosper. This led to the situation in which the Gardar farm failed while the Huls farm has continued to do well.