What is one key development that separates reptiles from amphibians?

There is more than one key development that separates reptiles from amphibians, and I do not think it is possible to single out only one key difference. In general, the reptile characteristics that differentiate them from amphibians are characteristics that allow reptiles to be less dependent on living near an aquatic environment. One development that allows for this is the reptilian skin. It is thicker than amphibian skin, and it allows for much less water loss through the skin; therefore reptiles can survive and thrive in much more desiccated environments. Another key development is the development of the amniotic egg. The amniotic egg is specifically structured in a way that allows reptiles to reproduce without being dependent on the aquatic environment. In the amniotic egg, an embryo develops structures that help maintain it within a terrestrial environment. This includes a fluid filled sac, called the amnion, that surrounds the embryo. Other structures help with gas diffusion, waste removal, and food sources.
http://projects.ncsu.edu/project/bio181de/Lab/animal_diversity/reptiles.html