The interesting thing about each new novel is that they all pose their own set of problems. Sometimes just setting things, often character interaction, odd plot bits and so on. Some come easy, some crazy-slow ... some never do work at all and get scrapped.
The most interesting part is that, for me, it's not "Oh, yawn, I'm working on another novel", but more: "Hah! I get to figure out how to write this novel now!"
I would post more on the subject, but I am tired now.