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.
First, there is the scene when Leslie, Maybelle and Jess return from Easter mess, They discuss faith and Maybelle says to Leslie: "If you don't believe in Jesus, you go to Hell when you die". Leslie dies less than a week later.
It Was His Sled: Leslie dies. Spoilered because it's not as well known as, say Darth VaderbeingLuke's father, but since it is one of the most discussed things about the story, it's hard to avoid hearing about it whenever the book or movie is brought up.
Jerkass Woobie: Janice turns out to be one. At the end of the film, it's implied she and Jesse may become friends.
Moe: Maybelle. Just ... Maybelle. This is even lampshaded by Josh Hutcherson and AnnaSophia Robb in their commentary, the former of whom calls her "just the cutest kid ever".
Moral Event Horizon: Scott and Gary cross it when they up their bullying of Jess following Leslie's death, Scott going so far as to outright taunt Jess over it in in class, complete with a smug smile on his face the whole time.
It ups the Nightmare Fuel when we find out that the Dark Master symbolizes Jess' father played by Robert Patrick, while the father himself isn't that bad of a guy, it does stir up some repressed T-1000 memories.
Paranoia Fuel: There's always a chance someone who is close to you could die in a freak accident.
Tainted by the Preview: Those who read the book knew that the story isn't a fantasy adventure, but rather the bonding experiences of two kids in a realistic setting. Seeing the 2007 film adaptation's trailers, which centered around the fantasy world imagined by the kids but now fully populated by CGI creatures, caused them to believe that the filmmakers completely missed the point of the book and were instead trying to make a Chronicles of Narniarip-off that the marketing strategy seemed to promote. In some cases, this turned them off from seeing the film altogether, even though the film was actually truer to the book than the trailers would lead one to believe.