Fridge Brilliance: Rabbits can't count past four. It's beyond the scope of a rabbit's mind to do the kind of mental counting humans can do, so presumably they use their four paws.
This one could make Sliding Scale of Gender Inequality a feminist example of Level 1: if a bunch of does had chosen to leave an established warren, they probably wouldn't have gotten into nearly as much trouble, as does are the usual dispersers among rabbits and could easily dig themselves shelters as needed, while picking up a few solitary males along the way. It's because Hazel's group is bucks-only that their journey and their quest for mates run into enough near-disasters to rate as remarkable.
Presumably, the White Blindness is a disease carried by ear mites—not literally fleas, but certainly a small biting parasite that lives on a rabbit's ears. (And really, are rabbits likely to care much about the difference?)
Fridge Brilliance: The appearance of cars, cigarettes, and other modern artifacts in what feels like age-old folklore can seem incongruous, until you recall the very short lifespans of the storytellers. To a rabbit, there have been hrududil on the roads for countless generations.