I'm not sure I would have 'gotten' this without having read the book a few times.
Max Schell (as Prof. David Malter) was basically wasted here, the relationship he had with his son Reuven just didn't come across nearly as powerful and moving as in the book. It's ironic that his most effective scene was with Danny in the library (and not any of his father/son scenes).
Robby Benson as Danny was pretty uninspired. For the most part, I felt like I was watching an actor play an autistic idiot-savant. The accent was weird, and his disengenousness was annoying. The best of his part certainly was the penultimate scene, when his dad (Rod Steiger as Reb Saunders) tells him he's OK with with Danny being a psychologist. (not even getting into how messed up his theory of child-rearing seems), but it was a powerfully acted, moving scene between the two of them.
In the end, I feel that Barry Miller (after a weak start) and Rod Steiger really carried the film. It can be difficult to see a beloved book made into an imperfect film (and nearly impossible to view dispassionately), but this was a pretty good attempt. Not every nuance came across, but it wasn't butchered, either.