This will be open to much in way of interpretation and opinion. I would say that what struck me the most in Darwin's work is how he frames evolutionary theory as something to happen over such a long period of time. Prior to Darwin, biologists never actively sought links between modern times and billions of years prior. Darwin helped to bring to light the change of organisms over a wide frame of reference. This and the idea of adaptation, that organisms adapt and appropriate changes to the environment in their own makeup, helped to bring to light a new level of scientific inquiry and thought process that helped to reconfigure how individuals see themselves in the world.