Sagan infamously begins this paean to the cosmos with, "The universe is all that was, is, or ever will be." This is not a statement derived from any of the sciences, not from Sagan's astronomy, not from physics, not biology, not from any science. This is a statement of metaphysics, the worldview of naturalism: there are no supernatural beings, the cosmos has no purpose, and there is no afterlife. From this unargued premise, Sagan extols the wonders of a godless world, with science (wrongly understood) as his church. And the historic church is, of course, the villain.
Sagan gives no indication that the theistic worldview (nature created by an infinite-personal being as good, intelligible, and worthy of harnessing for human betterment) of the leaders of the scientific revolution was vital to the development of modern science. Naturalism, however, has no reason to support the axiology or epistemology of scientific endeavor. Why think that unguided merely natural processes and entities would produce minds capable of knowing and improving the world? The naturalist, Eugene Wigner, even wrote famously of "The Unreasonable Effectiveness of Mathematics," given that there is no reason for the theoretical and intellectual realities of math to fit the world if there is no given order or meaning to existence.
So why would I give Sagan's work two stars? It is because he was a gifted writer, using the very gifts of God against God. Further, some might see through Sagan's naturalism and see into the Mind of God, as manifested in the cosmos.