Maybe I'll eventually write something lengthier about IT, aka Pennywise the Dancing Clown, aka Bob Gray, but for now I'll keep it simple.
The character of IT in the book is the ultimate monster. IT says it best with this quote: "I am eternal, child. I am the eater of worlds, and of children. And you are next!"
Skarsgard did a perfectly fine job with the material, but that IT is far from the eater of worlds. That IT was a jump-scare weirdo, a movie baddie that liked to jump out and scare the heroes, but never really did anything. In one moment IT would shake and jitter towards the camera, making you think you're gonna get got, then the next moment IT would slowly growl and snarl at the children, giving the heroes just enough time to escape or overpower IT.
And the ending? The kids beat IT with a baseball bat while IT just stands there, gaping and gawking, waiting for the next hit to come. In the book, and mini-series, they made silver bullets for the slingshot, and it was the kid's faith in their weapon that hurt IT. (Well, actually, in the book the kids went into The Void to perform the Ritual of Chud, and that's what hurt IT.)
Here's hoping Part Two will give us some actual terror.