While the natural history is reasonably close to real life, the book really isn't about rabbits. The author put a lot of effort into character development, maybe even more than he did in telling a story. I was an adult when I read Watership Down for the first time, and rented the video. I remember thinking that the movie was a bit too intense for small kids, it's nice to see that other people have the same sensibilities!
Watership Down is rather of a one-off for Richard Adams, in that it is peopled with rabbits. He has written a number of books, the couple that I have read are equally intense and disturbing. Some have even been banned because of their "adult" content.
On a much lighter note, did anyone else catch the gag in the Wallace and Gromit film Curse of the Were Rabbit, where the tune "Bright Eyes" (from the Watership Down movie) is playing on the radio?