Sauron being a literal flaming eye. It's a metaphor for how he knows everything that goes on. When Frodo is in the Wraith World, he says it's as if a great Eye was watching him, not literally.
Not offhand, but I haven't spoken to a lot of folks about LOTR in a long time.
I think the role of the elven rings is misconceived.
Then there's the idea some folks have that Gandalf and Galadriel are lovers. But I blame the Hobbit movies for that one.