The budding friendship between Leutnant Horstmayer and Lieutenant Audebert. Bear in mind that, despite the friendly ties between the two nations nowadays, at the time Franco-German enmity was seen as inevitable - almost genetic - which makes their amity that much more touching. In particular, this little exchange.
Horstmayer: Once we invade Paris, the war will be over, and you can invite me for a drink at your house.
Audebert: You know, you don't need to invade Paris to have a drink with me.
Of course, at the end of the film, Horstmayer reveals that his wife is French, so he was defying the "Erbfeindschaft" from the beginning.
Marriage among the Germans, French, and English, even back then, as now, was actually relatively common (one only needs to look at the British royal family to be reminded of this). This revelation serves only to underscore the absurdity of the "they are different" propaganda.
Though almost all the characters are fictional, the events depicted in the film really happened. And not just in one place. And not only on the Western front, either.
Spink's expression when he hears the bagpipes. His look of childlike wonder is enough to melt anyone's heart.
After a terrible argument between Lieutenant Audebert and his father, a general, the lieutenant reveals he has a son named Henri. The general, while disagreeing with his son over the war, tells him "Let's both try and survive this war for him.".