Eustace realizing the full implications of being turned into a dragon, specifically, how utterly dismal his life will be. When he first realizes what happened, the thought jumps to mind that now he has the power to get even with his cousins and the others on the ship for his perceived ill-treatment. However, he quickly realizes that he doesn't want vengeance; he wants their friendship, and now that's impossible:
He realized that he was a monster, cut off from the rest of the human race. An appalling loneliness came over him. A powerful dragon, crying its eyes out under the moon in a deserted valley is a sight and a sound hardly to be imagined.
Reepicheep sets off alone for Aslan's country. For him, it's a Happily Ever After ending, but for the reader, it's very bitter to say goodbye to such an awesome and funny Talking Mouse.
At the end, Aslan tells Edmund and Lucy they're too old to return to Narnia. Ouch.
Lucy sobbing as she bids goodbye to Caspian and Aslan.
The trio's reactions after landing back in England. Eustace has a half-smile on his face, clearly thrilled with the adventure he's just experienced ... but then the camera cuts to Edmund - who is simply staring blankly at the painting - and Lucy, who looks like she's barely holding back tears. Eustace sobers up at once and hangs the painting up again, almost reverently, on Lucy's mantle. He's clearly a far cry from the bratty child who'd threatened to smash it at the beginning of the movie.
The final scene, as we see the Dawn Treader disappear over the waves in the painting, followed by the Door-Closes Ending. Made even worse by Eustace's narration and Lucy's last Longing Look as the world she loves so much closes on her ... at least for the time-being.
Eustace: We spoke often of Narnia in the days that followed. And when my cousins left, after the war was over, I missed them. With all my heart. As I know all Narnians will miss them, until the end of time.
There's also Lucy's nightmare where her wish to be as beautiful as Susan has her become her sister, and discovers that she never existed and her siblings never knew Narnia as a result. When Lucy wakes up, she looks so horrified and ashamed as Aslan asks, "What have you done, child?"
