The deaths of Thorin, Fíli, and Kíli, and particularly the Final Speech of the former:
"There is more in you of good than you know, child of the kindly West. Some courage, and some wisdom, blended in measure. If more of us valued food and cheer and song above hoarded gold, it would be a merrier world. But sad or merry, I must leave it now. Farewell."
- Fíli and Kíli dying is understated in itself, but throughout the book the narration makes it clear that they're the youngest of the group, which, when you know about how Tolkien fought in WWI, makes the terrible waste of young men's lives all the more poignant.