As a literary device, though, the idea of an easy-going hedonistic society with a nasty dark secret everyone would much rather not talk about, has at least one precedent: the Eloi in H.G. Wells' The Time Machine.
And the main thing that Watership Down brings to my mind is the spoof promotional slogan someone thought up after the movie version came out: "You've read the book, you've seen the movie; now eat the pie!"