The Lego Turtle in Georgie's Bedroom
One major difference from the book is the total exclusion of what is known as the Macroverse. It's basically the alternate dimension where It actually lives. When Bill goes to the Macroverse, he meets a giant, ancient turtle named Maturin (yes, really). Maturin is the creator of the universe, and also gives Bill advice on how to defeat It.
Right before Bill sees Georgie's ghost in his home, he goes to Georgie's room and picks up a little Lego creation that's shaped like a turtle. It's surely a hidden reference to Maturin. The Losers also talk about seeing a turtle underwater when they're swimming together!