As entertaining and informative as this book was, with especially effective editing and translation by H.A. Giles, I don't know that I have much insight to offer into it. The natural approach is to hold it alongside folk tales from other cultures, especially those compiled by Italo Calvino, and see what the overlay reveals.
Doing so, it immediately becomes clear just how universal the human approach to the supernatural is. Among the themes and plots that recur with startling frequency in both volumes:
The young man of poor means but natural talent and good intention, who is treated unjustly but ultimately receives his due with a little help from the other world.
The girl who is treated badly by a mother figure to whom she has no blood ties, but endures out of an abundance of filial piety, and ultimately receives her due with a little help from the other world.
The lover who is not what they seem, but is all the more devoted and loving for it (although one interesting divergence is how often these figures were male in Calvino's stories, and how often female in P'u's).
In fact, dressed in different clothes, any story from the one set would be right at home in the other. The virtues rewarded are the same: fidelity, industry, honesty, patience; as are the vices: greed, bad faith, impiety. I'm sure a better and more comprehensive comparison than I am equipped to perform exists, and draws conclusions from its findings. As for me, I am in the mood to take a significantly less temporal approach, and go out into the woods to find a fox spirit or fairy of my own to ask.