What are the most important legal and financial steps for new parents in the US? For example, establishing a will, purchasing life insurance, setting up a college fund, etc...
Life Insurance. You have someone (your child) now who is completely and solely dependent upon you to take care of them. If something unfortunate were to happen and you were no longer there to provide for them, that would be tragic. Life insurance, like any other insurance, is a necessary evil for parents of young children. Tern life insurance is typically quite affordable in the long run and provides a financial security blanket.
I also agree with all of @Sarato's comments regarding Legal items.
Here is a list based on our experiences, and what I was able to research:
Financial / Medical