These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
John Thornton: As you love me, Buck, as you love me.
Gary Stu: Buck, while at first a rather normal dog, slowly evolves into this within the course of the book. He becomes smarter and more cunning and more brave and more loyal than any other dog : essentially, he has no flaws by the end of the book. After a while, you begin to wonder about the nigh-constant stream of praise both London and the other characters feed Buck. Though to be fair with London, he does gets some character development and goes through realistically brutal trials to become as incredibly Badass as he is.
God-Mode Sue: At the end of the book, arguably considering he took out an entire tribe of Yeehats without a scratch on him.
Nightmare Fuel: The idea of getting overwhelmed by forty dogs and torn to pieces ala Curly if you so much as fall down.
Tear Jerker: Everything leading up to Buck leaving John, and then Buck coming back to find John's dead body.
What an Idiot: Hal, Mercedes and Charles. Just about everything they do is stupid and they're frequently told as much by the other Alaskans. They all pay for it in the end.
What Do You Mean, It's Not for Kids?: Because the story is short, the language is pretty easy, and the protagonist is an animal, this often ends up on kid's shelves and read in middle school. It's actually an extremely bloody story and not at all politically correct when it comes to race and gender.