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.
Adaptation Displacement: The MGM movie adaptation is much better known than the Oz books. Some of the more recent sequels to The Wizard of Oz — such as Wicked — are based on the movie and not the book version.
Big Lipped Alligator Moment: 'The Dainty China Country' chapter in the original book features Dorothy and her companions finding a town where the houses, animals, and citizens are all made out of china. Some, such as the accident-prone clown, Mr. Joker, have been broken and mended several times. Nothing of any relevance to the story happens here — the china citizens neither help nor hinder the protagonists — and after Dorothy and her friends leave, it's never mentioned again.
It's more or less accepted that it only exists to lengthen the journey from point A to point B. It does make you wonder why they bothered to go over the wall instead of around it, as would be sensible.
Broken Base: The fandom is largely split between fans of the books and fans of the MGM movie. And that's not counting residents of the former Soviet Union, whose experience comes mostly from Volkov's adaptation, Tales of the Magic Land.
Fanon Name: Other authors have given the names "Elphaba", "Evillene" and "Bastinda" for the Wicked Witch of the West in their versions of Oz.
Faux Symbolism: So much. There's a reason some historians see it as a Farmer's Movie. To name a few for the history buffs, Dorothy has silver shoes and walks on a yellow brick road to get to the Emerald City, Dorothy ends up missing the Scarecrow the most... Though it's all highly debated, and not everyone believes it. See Epileptic Trees
Nightmare Fuel: How the Tin Man got his parts, or rather, how he lost them. Also, when he meets up with his disembodied head. His former and sentient disembodied head; as he has a tin replacement now.
When the Wicked Witch commands her airborne simian lackeys to destroy the Tin Woodsman and the Scarecrow and Dorothy and make the Cowardly Lion into her prisoner.
Older Than They Think: Movie fans are often surprised to learn that The Wizard of Oz is based on a book that was published in 1900 and the book had sequels published over the span of 50 years.
Sequelitis: Baum wrote thirteen more books about Oz. While some were quite good, a lot just got very formulaic.
Big Lipped Alligator Moment: The visit to the land of the Cuttenclips that takes up most of episode 44 and has no effect whatever on the rest of the plot. (It's a holdover from the book that section is based on, The Emerald City of Oz, which includes an extended sequence of Dorothy visiting various strange lands in and around Oz.)
Complete Monster: The Nome King. Aside from being a Psychopathic Manchild and The Caligula, he keeps a collection of people-turned-ornaments in a state of And I Must Scream, one of them being the captured Prince of Ev. After Dorothy and her friends thwart the deadly ornament game, The Nome King then tries to burn her and her friends alive with magma on several occasions. Oh yeah, he also wants to take the Tin Man and Tik Tok apart.