The idea of getting overwhelmed by forty dogs and torn to pieces a la Curly if you so much as fall down.
The starving dogs. A group of dozens of dogs, some of whom were probably loyal sled dogs left or lost to the wild, come crawling silently into the camp as skeletal animals completely insane and hell-bent on eating everything. On top of that, they basically win despite many of them being killed because the team just couldn't stop them. Not long after this, the quiet and gentle Dolly snaps with a blood-curdling howl and runs mad, chasing and terrifying everyone in camp until she is finally killed with an axe.
Curly's death. The big, sweet, friendly Newfoundland is savagely ripped apart for the mistake of trying to befriend a strange pack.
Despite pushing his way back into his position on the sled after collapsing, Dave eventually succumbs to exhaustion and has to be given a Mercy Kill.
Hal, Charles and Mercedes are more stupid than malicious, so it still manages to be a little sad that they fall victim to their own inexperience. But the reader is more likely to feel sorry for the rest of the sled dogs we've come to know, who fall through the ice and die with them.
Everything leading up to Buck leaving John, and then Buck coming back to find John's dead body.
Values Dissonance: The depiction of the natives as bloodthirsty, unreasoning savages. Also, naming a black dog Nig.