Night of the Lepus is a heroic, but fatally-flawed play on your basic "critters affected by human-induced environmental pressure (usually radioactivity-related) become very, very large" B-movie plot. In this case, however, the producers opted to tackle a real challenge: instead of making the monsters something that people are already scared of when normal-sized -- spiders, ants, worms, teenagers -- they went for a beast that hardly anybody fears.
Gigantic, omnivorous, surly, aggressive -- and, of course, prolific -- rabbits.
Via trick photography, miniature sets, and the occasional use of what I can only describe as a "hot grill" (which, when electrified, caused the thundering herds of bunnies to leap sideways and shake their paws), the director failed utterly to convince anyone, including the performers, that the giant rabbits were (a) frightening or (b) there. But the attempt itself was so audacious that I have to give the filmmakers credit; yes, it was a failure, but what a ballsy failure!
Personally, I think the big mistake was in making the rabbits, uh, big. Aside from the fundamental problems of getting a large population to become so, uh, large, without attracting any notice until it was too late (a problem handled with varying degrees of success [from "zero" to "well, OK, I'll bite"] in other giant-critter movies), there's the basic difficulty that cottontails, even when they're the size of tractors, don't look very frightening. And they don't act very frightening. (Unless they're standing on the "hot grill".)
Now, as I'm sure some of our Australian friends could tell us (or, for that matter, anybody who's tried to raise lettuce anywhere near a rabbit warren), rabbits -- perfectly normal-sized rabbits -- are capable of causing immense destruction. And I can personally attest to the potential fear factor of confronting a number of annoyed jackrabbits on a cold winter's day; they'd each picked a cozy spot under the horses' grain trough, and they clearly intended to wait there for the inevitable spillage, no matter how close I got. But they glared at me with those big yellow eyes -- jackrabbits have eyes like owls or hawks, totally unlike the soft brown eyes of cottontails -- and convinced me that I didn't want to find out what they'd do if I made them move.
So my humble suggestion to any budding movie-makers out there is, when attempting to generate fear via critters normally considered cute and furry, don't immediately leap (sorry!) to the conclusion that bigger is scarier. Find out what they do that's annoying or destructive, and play on that; induce menace via increasing numbers, not increasing size, or via disturbing changes in behavior, perhaps a growing group consciousness. A remake of The Birds, for example, using lean, muscular jackrabbits with cold yellow eyes, could have instilled an entire generation with a terror of furry, hopping things!
But that film is yet to be made. Night of the Lepus is not that movie.