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.
On the Sliding Scale of Gender Inequality, this could be 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?)
It's myxomatosis, which actually is transmitted by being bitten by fleas or mosquitoes that have fed on an infected rabbit.
In the Sandleford warren, cowslips are considered to be a rare treat, to the point where some Jerkass Owsla bully Fiver away from one that he found just so they can eat it themselves. However, the cowslip plant is fairly toxic to rabbits. So when a rabbit who goes by the name Cowslip invites the protagonists over to his warren, it should be your first clue that something is very amiss with it.
It's not just Cowslip. Hazel is curious that one of the rabbits in Cowslip's warren is named Laburnum, a name that translates into Lapine as "Poison Tree" (laburnum is toxic to rabbits and other animals). Some of the other rabbits in the warren are Silverweed and Kingcup—both of which are also plants poisonous to rabbits. (Unsurprisingly, the one rabbit they end up keeping with them is Strawberry; strawberries are fine as occasional bunny treats.)
Fridge Brilliance/Horror: Cowslip's community have largely abandoned the practice of telling tales of El-ahrairah. Superficially, this demonstrates how their constant stress and victimized dependency upon humans is destroying their culture. But the tales in question have a purpose: to keep rabbits' survival instincts keen, both intellectually (reminding them to be wary and full of tricks) and viscerally (because most of the stories are scary and stimulate listeners' fight-or-flight reflexes). Any members of the warren who did keep telling the stories would've retained their gut-level rabbit instincts better than the ones who gave up the practice ... and those very survival instincts are what got the story-lovers killed. What do a rabbit's survival instincts urge it do if it's startled? Dive for the cover of underbrush. Where does the Man place the shining wires? In the underbrush.
Fridge Horror: General Woundwort's (presumed) death may have been a very good thing for humanity. By the end he was up to "dogs aren't dangerous", and actually almost succeeding in fighting them off. What's the next logical step?
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.
Fridge Brilliance: The fact that the doe at the end is ostensibly telling a story about El-ahrairah, but is actually telling Hazel, Fiver and the others' story makes you wonder how many of El-ahrairah's stories actually are things he did, or if he ever really existed at all.
Or if he has existed thousands of times — a gestalt of every truly brave or ingenious Chief Rabbit.
All but explicitly called out in Cowslip's Warren. That warren has no chief rabbit because "A chief rabbit must be El-ahrairah to his people and lead them from danger, but they have no danger except for one."