One problem with world-building like his (though you see the full extent only in the Silmarillion), is that it leaves no stray corners.
Tom Bombadill doesn't fit. Neither does Ungoliant. Having laid out the metaphysics of the world, he has no place where things, either wonderful or horrible, that do not have explanations for them.
It's interesting to read his earlier versions, where things are much less clearly delineated, and see the effect it has where even he doesn't know. An effect like, but not the same, as The Lord of the Rings, where the things are laid out in Tolkien's mind, but the hobbits don't know them.