I've been toying with the idea of visiting Paris and Italy in November so it's nice to hear your take on this. I would go in with the attitude the weather will become ambiance rather than something to regret. We typically go in the Spring and have thus far been lucky with fewer crowds than the peak seasons.
Off season is the way to go. 40°F might not be so great if you live in the Southern U.S. So far on this trip, highs have been upper 50s-mid 60s.
Yesterday, Naples and Herculaneum, a town/city destroyed by the same eruption that got Pompeii. We've been to Pompeii before. Pompeii is huge! This is much smaller, but just as cool. There is an area which is at sea level where there are 300, 2000 year old skeletons, people who were trying to escape dying, by hiding in storage areas by the water. This area is in a hole 40 feet down that was covered by mud and lava. In fact the entire town was under 10-15 meters of debris that flowed down off Mount Vesuvius before it was discovered in the early 1700s. In the panorama shot, on the right side you can see it's in a hole.