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.
Fridge Horror: After frantically searching for the clover, Horton is described as being "more dead than alive". When you read the book as a kid, you think nothing of it...but when you happen upon it when you're older, you suddenly think "Oh my god, is he about to diefromexhaustion?!"
And in the movie: Katie. Seriously, some of the faces she pulled will haunt your dreams.
While in the animated special, the Wickersham Brothers singing their song can be seen as eerie.
Especially when their song references McCarthy's campaigns.
Tear Jerker: Horton finds the clover in the movie, and for a few seconds, no answer. The wail of despair he lets out is just wrenching.
The Scrappy: Yes, the Kangaroo's the main antagonist, but it seems like she's going out of her way to be the most unlikeable character, be it the book or the movie. Seriously, inciting a mob to torture Horton just because he won't give you the satisfaction of making you think you're right?
Not to mention when Vlad suggests she give him Rudy to eat in exchange for his services, while she decides against it, she still has to think about it for a second.
What Do You Mean, It's Not Political?: Pro-life activists think this book was anti-abortion. Not only was Dr. Seuss pro-choice but the book was published two decades before Roe v. Wade. According to his wife, the central allegory is about fascism, corporatism and capitalism — "a person's a person, no matter how small" refers to big shot governments and businesses stepping on the common worker. At one point a pro-life group actually tried to use the line as their slogan, until Mrs. Geisel sued them out of them.
It's also an allegory for how the Japanese were being treated after WWII; the book is even dedicated to a Japanese friend, Mitsugi Nakamura. There's shades of The Atoner here, as during the war Seuss did his share of anti-Japanese propaganda cartoons which he came to deeply regret.
It works as an allegory for Isolationist foreign policy too.
The Woobie: Rudy, the little baby joey that has to stay in his Jerkass mother's pouch. It's hard not to feel sorry for the poor fella.
Jojo, who is so afraid of letting down his dad that he took a vow of silence.