Author Phobia: Stephen King wrote an essay wherein he described the "ten bears" of various common fears. Rats, the dark, squishy things, closed-in spaces, things from outer space, fear for someone else, and a few others were all on the list. He pulls out all the stops with the creation of It, since It is a shapeshifter that takes on the form of the victim's greatest fears.
Insistent Terminology: The Swedish translation of the book uses this for the color orange. The color orange is called "orange" in Swedish while the fruit orange is called "apelsin". Whenever anything orange appears in the novel it is always referred to as "apelsinfärgad" - "apelsin colored". This has the effect of associating the color with It, giving it a much creepier vibe.
In the English version, orange is associated with It, as well. In every encounter, the Losers discuss how It leaves orange pom-poms. Much creepier than it sounds.