Prophecies for one. Above and beyond their cliche status, they can be very tiresome in a manner that no lack of repetition could cure: if the prophecy comes true in the obvious way on the face of it, the plot looks railroaded. Prophecies need to come to true in outlandish ways, or be brought about by the actions meant to prevent them. Oedipus thought he was evading the prophecy that he would kill his father and marry his mother.
Gods, even if they aren't handing out prophecies, need purposes of their own but those purposes, and what their judgments are, ought not to be too clear to mortal minds. Even the nymph of a spring.
Magic whether in spells or in items. Powerful and sensitive to context objects ought to act in ways the characters, even their owners, can't predict. (If it's intelligent, it needs a mind of its own.) Spells should not be useful on acquiring them, only when the character figures out how to use them prudently.
Which helps give the story its final elegance: to be foreshadowed but clearly so only in hindsight.