NB: Spoilers ahead. I can't be bothered with multiple spoiler tags, so skip this if you haven't read it.
I was surprised that no-one had mentioned the Bev factor as well. Although, I didn't mention it either, so there you go.
As for the turtle (or, The Turtle) I always got the impression that he/she/it was a god-like entity, but one without much vested interest in the world of humans. Didn't he accidentally vomit up the universe? I haven't read it for a few months, and I always zoom through the last section, so I may be off the mark here.
Also, isn't there talk of a higher power than the turtle and It? Something that lives beyond the edge of existence where Bill almost falls? Arrgggh, this memeory of mine. I'll have to read it again.
I have to say that some of my favourite sections of the novel are the Derry interludes where Mike discusses the history of the town. The disappearances of the entire population, crazy woodsmen going to town with an axe, dads eating poison mushrooms. The fire at the Black Spot is fantastic. I'd read it as a short story in its own right.