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.
Anticlimax Boss: Every big threat in The last Olympian. The Drakon dies in six pages, and Kronos barely fights enough to look strong. Even in the last fight it didn't take too long to bring him down. Even Typhon.
Especially Typhon. At least the others had actual battle scenes where the hero had to struggle. Typhon is played up as the big monster crushing the majority of the Gods for the entire book and then he actually shows up and is taken down by Poseidon's army in a few paragraphs.
Crack Pairing: Tons. Rachel/Nico, Thalia/Nico, Percy/Clarisse, Rachel/Luke, Apollo/Hermesnote Which is actually technically possible in the source material, especially given the fact that both are involved in same-sex relationships with mortals and other gods, etc.
Critical Research Failure: In the films, everything involving Persephone, Hades' wife. In the original myth, Hades and Persephone have a close, loving relationship, or at least better than several other marriages between the gods. More noticeably, the fact that she is present along side Hades during the story, which takes place during the summer. Persephone, however, only spends winter in the underworld, and her mother Demeter misses her, which is why the world freezes over every winter. In the book, Persephone is mentioned in passing and her throne is seen beside Hades' own, but she herself is never seen as goes the story.
There are a number of people who can immediately guess that Bianca and Nico di Angelo are Older than They Look in The Titan's Curse, when, to evade their pursuers in Washington, D.C., they duck into the Metro. To Bianca, the Metro is new to her, and she even mentions that the last time she and Nico came through D.C. (which was no less than a year prior to the events of the story), the Washington Metro did not exist. Just for the record: the story is set in 2007. The di Angelos are in their early teens. The first segment of the Washington Metro system opened in 1976, well before either Nico or Bianca would have been born. It Makes Sense in Context though, once Percy deduces that they spent time in the Lotus Hotel & Casino. The Last Olympian reveals that they were actually born sometime in the mid-1930s, and put in the Lotus Hotel sometime around 1941.
It might or might not be intentional, but the mention of a statue of Susan B. Anthony strangling Frederick Douglass in "The Last Olympian" might be a roundabout reference to how the two civil rights leaders, formerly steadfast allies, became divided over the issue of the Fourteenth Amendment allowing Black males to vote, but not women of any race (and to add insult to injury, it was the first Amendment that explicitly mentioned male and only male citizens).
Growing the Beard: Most agree that The Titan's Curse was where the series really began to show its quality.
Fan Dumb: Some fans ship Zoe and Artemis, despite the fact that it is specifically stated that the Hunters give up ALL romantic love. Although it is understandable that they want homosexual representation in the series, they are ironically hurting their own cause because if this were true it'd be implying that homosexual love "doesn't count" and "isn't real." Think it through, guys.
The oath states "I pledge myself to the Goddess Artemis. I turn my back on the company of men, accept eternal maidenhood, and join the Hunt". They have room to play around with its exact meaning.
Fan-Preferred Couple: While Percebeth is still dominate, there are still rather decently sized fandoms for Percy/Luke, Percy/Nico, Percy/Artemis, Apollo/Hermess, Poseidon/Athena and Percy/Thalia
Foe Yay: Luke and pretty much everyone. Particularly Percy, Thalia, Annabeth, and (if you squint) Grover.
"You will fail to save what matters most in the end." Congrats Percy, you and Annabeth may be relatively safe together in Tartarus, but Gaea now has the materials to set her plot into motion. You have doomed the world by choosing true love over it.
Promise (which seems to be the Arc Words in Luke's life), after reading The Diary of Luke Castellan
The tense relationship between Nico and Percy becomes downright depressing after House of Hades, where it's revealed he was hiding his true feelings from him.
Ho Yay: Percy and Nico share a bit of this in The Last Olympian. Luke and Percy do, too, particularly in the first book.
And then in later books, it turns out that Nico actually has/had feelings for Percy.
Inferred Holocaust: In the first book when Percy, Annabeth, and Grover face Hades, the god of the dead becomes so enraged that he causes not one but two earthquakes, and Percy remarks that it will not have been a peaceful night in L.A. When the kids resurface, Los Angeles is burning. No casualty numbers are given, but it could qualify as a "Funny Aneurysm" Moment, as Hades had just finished complaining about how the Underworld was overcrowded and didn't need more subjects. Kinda ironic.
Manhattan sounds pretty bad by the end of the last book. Everyone had fallen asleep, which probably includes people who were cooking things, people who were in the hospital getting surgery, etc. And Fifth Avenue is a disaster.
Les Yay: Silena and Clarisse, Annabeth and Thalia, ZoŽ and Bianca.
ZoŽ and Artemis, one-sided on ZoŽ's part, arguably.
Clarisse and Silena in particularly considering they play out the relationship between Achilles and Patroclus to its tragic conclusion. And we all know about them.
The Untwist: Percy's origins. Anyone who knows anything about Greek myth could see the title of the series, the cover of the first book, and then put two and two together. It's really not that hard.
The fulfilling of the prophecy in The Titan's Curse It says "One will be lost" and everybody assumes it means someone will die. But it never outright states dying, so with the usual Prophecy Twist you could expect Bianca to be lost and later found. It turns out she gets Killed Off for Real.
The Woobie: Everyone not named Gabe, Ares, or Kronos.
Luke's mother, May Castellan, also deserves a special mention here.
In The Last Olympian, the narration actually says that Hermes "looked like he needed a hug." If that's not baiting woobiedom, nothing is.
MRS. JACKSON. Ridiculously nice, sweet, and caring, and yet gets get stuck with an abusive slob of a husband and serious financial problems. Luckily, things get much better.
Even Dionysus has a little of this after he revealed his relationship with Ariadne to Percy in the third book, and after one of his sons die in the fourth book.
Tropes which only apply to the film:
Angst? What Angst?: Percy seems to handle the sudden death / disappearance of his mother very well...
Anti-Climax Boss: Kronos in Sea of Monsters. Makes you wonder why they even bothered to bring him in this early.
Love It or Hate It: The Sea of Monsters film. Some thought this movie was just as bad as the first,others thought it was better and more entertaining.But mostly everyone agrees that it did follow the book better than the first one did.
Nightmare Fuel: The scene in Medusa's shop. While it isn't that creepy in the book (there the scene is a lot less ominous up until the Uncle Ferdinand bit), the movie added a panicking woman whose husband had already been turned to stone and she's hysterical with fright. She eventually gets turned into stone too, and the only mention that's made of it is when Annabeth explains how she scraped her wrist (the woman had been holding her wrist when she had been turned to stone and Annabeth had to pull her hand free).
They Just Didn't Care: Similar deal (unfortunately for fans of the book) to The Seeker, considering how the director admitted to not even reading the first book. So much expectations were put into the movie....only for the fandom to be gravely disappointed. The only similarity between the film and the book is the main plot line (just barely).