I saw the movie for Atonement years ago (when it first came out) and have sort of intentionally waited to read the book so that some of the plot and imagery were out of my head. I have read some of McEwan's other stuff and tend to like him and was excited to see what the book offers that was not as evident in the movie.
I think that the single biggest difference between movie and book is how much more sympathetic Briony appears in the book. In the movie, she comes off (as I remember) as a spoiled brat who is angry and bored and sort of invents a story to entertain herself. In the book we see the adolescent struggles of a young girl trying to understand the adult issues in which the older girls (Cecelia and Lola) are already enmeshed.
Not only are the older girls wise in a sexual way (although certainly the book leaves no question that Lola is simply raped twice in one day by her future husband), they are also more sophisticated in their manner of expressing derision through acquiescence: "And yet, Briony struggled to grasp the difficult thought, wasn't there manipulation here, wasn't Lola using the twins to express something on her behalf, something hostile or destructive? Briony felt the disadvantage of being two years younger than the other girl, of having a full two years' refinement weigh against her" followed by, "it was Lola's briskness, her obliviousness to anything beyond her own business, and Briony's certainty that her own feelings would not even register, still less provoke guilt, which gave her the strength to resist."
As Briony develops intellectually (rather abruptly through the course of a single day in the novel), she moves to question her understanding of other-ness as also incorporating a self. This reminded me of a quote from David Foster Wallace about children believing that the world springs into being as they awake in the morning: "Did her sister also have a real self concealed behind a breaking wave...Did everybody..? if the answer was yes, then the world, the social world, was unbearably complicated, with two billion voices, and everyone's thoughts striving in equal importance and everyone's claim on life as intense, and everyone thinking they were unique, when no one was....It wasn't only wickedness and scheming that made people unhappy, it was confusion and misunderstanding; above all, it was the failure to grasp the simple truth that other people are as real as you" Ironically just a few hours after this thought, Briony takes away Robbie's individual hood both by literally sending him to jail and by confusing his figure with that of Paul Marshall.
I loved the parallel when Turner watches an RAF man being tortured on the beach in France between his own arrest. Certainly, Lola stood passively by while Briony attacked him (as Turner does on the crowded beach); it takes a brace man (Mace) to "rescue" the RAF man (similarly to the way the Cecelia rescues Robbie.
I liked that the rejection letter that Briony gets about her story of the two figures at the fountain (clearly the day that Cecelia and Robbie fought) contained references that were not in McEwan's text. I wondered if this was because Briony's story is an imperfect representation (which of course it is a story within a story and our "true" account is that which is presented first and written by McEwan) or if McEwan's original draft received some criticism that resulted in the final version that we read. Just a fun post-modern look at itself from within the story.
Overall it is a compelling book (solid plot) with some interesting psychological comments.