I found the music score really striking (usually I barely notice it) since it would take sounds from the film, like a typewriter tapping, and weave it into the music.
I also re-watched it with the commentary by director Joe Wright. I like watching a film with the commentary to find out more about the construction of a film (Aside: I think this harks back to when I was young and always enjoyed books like Children of the New Forest and Swiss Family Robinson which have a large component of "how to do things") This was a particularly good one, talking not only about the technicalities, but bringing out themes and social background that were not immediately obvious. There was a motif of 'swallows' that I hadn't picked up on, but one of 'water' that I had.
At one point the young girl, Briony, is pictured looking out of a stained glass window of St Matilda, who turns out to the patron saint of falsely accused persons. During a scene between the two lovers in a tea-room, the director points out that the upper class accent of that time uses short vowels, and vowels are what are used to express emotion, so they have to express their emotion by very subtle movements.
Although I was moved by the story, the craft of the film was what impressed me most. They had a relatively modest budget and tight shooting schedule, but the amount of care that was taken made it seem like they'd had months to get every detail just right.