A novel's prose has to be good enough to get me into the story. I'm most aware of writing for the first two-thirds of a novel. From there, if the writer has set up a climax compelling enough, I care more about seeing what happens than about the finer points of the writing.
I also think the closer the narration is, the more the writing matters. If you're telling a story without much psychic distance--first person or third person-limited--you have to get into the viewpoint character's mindset and voice. The prose can't be very clunky. If you're telling the story from a greater distance, you only need to give the broad strokes. I'd argue that it's more difficult to love a broad-stroke character, but they are readable enough.
I admire some writers' prose more than others, and I prefer certain narrative styles to others, but these differences keep me fascinated about writing overall.
I've come to think Ian Fleming's prose was unremarkable and sometimes stiff, but I'd reread DIAMONDS ARE FOREVER and FROM RUSSIA, WITH LOVE anytime.