To me one of the more interesting points of this Appendix, when I first read it many years ago about the same time the Silmarillion was first published, was that (correct me if I'm wrong about this) it makes the only mention of Eru in LOTR "The Valar laid down their Guardianship and called upon the One,..."
That suddenly gave Tolkien's world a whole different complexion, that it was fundamentally a monotheistic one--well, maybe it would have been obvious if I had been older or known more about Tolkien's life at the time.