I live in California, so I don't get the harsh winters that some of you guys get. Well, we do get several days of snow every year, but our lows only get into the 20's usually.
Anyway, my coop has a dirt floor and I love it. I can clean it out with a rake and spray disinfectant right on the dirt. I usually keep straw on top of the dirt in the winter, but in the summer I don't put anything down. I also haven't had any problems with mites or anything like that.
Around the outside edge of the coop and run I dug a small ditch (about 6 inches deep and 4 inches wide) and put post hole cement in it. You know, the kind you don't have to mix with water. Predators are unable to dig past it. We have raccoons, coyotes, wild boar, rats, mice, foxes, bobcats, wild dogs, etc. We can see where things have tried to dig in or pull the wire apart, but nothing has been able to get in.
Hope this helps and good luck with your coop,
Lori