I am driving from Zurich to Venice. What kind of kind of driving conditions can I expect in mid December, especially through the mountains? Are the roads well maintained?
Unless Italy is going to be hit by a crazy blizzard you should not have any problem. Streets in general are in good condition, especially in the north, and they are kept clean in case of snow.
Beware of the fact that winter tires or tire chains are mandatory in many roads during winter and this is especially true in Veneto where from November 15 to April 15 your car has to be equipped with winter tires or you must have tire chains on board (not just any type of chain but ones that suit your tires). The fine is between €84 and €318. Depending on the particular route you are gonna take this may be true for other roads as well.
As most of it is highway, you can expect normal road conditions without any problems. Unless it snowed heavily and very recently, there shouldn't be any snow on the roads and the roads are usually well maintained both in Switzerland and Italy (at least the highways). Winter tires are recommended though.
To use highways in Switzerland you'll have to buy a toll sticker for 40 francs and in Italy you'll have to pay toll depending on how far you drive.