Of barrow-wights and the Balrog: Tolkien brings horror to Middle-earth

Much is made of the differences in tone between J.R.R. Tolkien’s The Hobbit and its eventual sequel, The Lord of the Rings.  One was written for children, the other clearly was not.  But what is the defining characteristic of Tolkien’s epic (and there may be more than one) that sets it apart from its child-friendly origins?

I’d venture to say it’s that Tolkien brings horror to Middle-earth.