In his introduction F. F. Coppola says he had expected The Godfather to be more classy, i.e. literary. He got something more akin to a dime store novel.
Like Coppola, I too expected something classy, so at first I was a bit disappointed; Puzo is not a great writer. He repeats himself often, uses dialogue for info dumps, etc. But if you keep in mind the pulp fiction aesthetic and let the story take center stage, it eventually earns its stripes.