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.
Ear Worm: The main theme: "Ghoooostwritaaaah! Word!"
Don't forget the music that played whenever they rallied.
Ensemble Darkhorse: Victor, Rob's friend in Building Bridges, seems to be very popular in the fandom. The producers must have noticed because he shows up in another episode despite not being particularly involved in the plot.
Calvin Ferguson, both with those who love to hate him and those who root for him.
Fanon: One of the leading theories about Ghostwriter is that he's the ghost of Jamal's great-grandfather Ezra.
"Did you actually think that Rob would hook up with you?"
Fridge Logic: In the "Lost in Brooklyn" episode, why didn't the girl simply call a taxi and ask her dad to pay? Well, it's easy to assume that since she was from Mozambique that she never used a taxi before and her dad forbid her from entering strange cars. That and she was 13-15, they're not always going to think rationally.
Ho Yay: Hector has a pretty blatant crush on Avatar, the male makeup artist, when the team appears in Lenni's music video.
Not to mention the guy himself. He's a male makeup artist, and he calls himself "The Fabulous Avatar."
In "Lost in Brooklyn, Part 1" Tina meets Safira, visiting from Mozambique. Ostensibly, she's interesting because she's foreign. Everyone does rush to see who it is coming to school in a limo, but Tina's the only one who takes one look and says "Oh, wow!" when Safira gets out of the car. By the time we cut to the classroom, everyone else has long gotten over it all, but we have Tina looking around the room until Safira is introduced by the teacher, whereupon Tina instantly stops moving and gets this big, goofy, ear-to-ear smile, and watches Safira's every movement with said smile until she sits down next to her. Meanwhile, everyone else is acting like your average bored schoolkids. When she gets to say more than hi, she talks fast and rambly, and at one point talks about trying talking to her (a second pitch at being in her video, actually) with lines that sounded very much like someone trying to ask out a crush. The entire first part is driven by her need to get this girl to be friends with her, and there's master plans and talk of "destiny" that she wound up sitting next to her and on and on and it doesn't end up having to do with the video. There ends up being a picnic in the woods on a blanket, and a Death Glare at Safira's dad until he leaves. The mystery of the arc is introduced only in the closing moments of part one; the rest is entirely Tina positively drooling over this girl and stopping at nothing to get her attention. (Well, that and a few minutes devoted to Hector's introduction, setting up Rob's replacement.)
Jerkass Woobie: Many of the villains are so pathetic that it's pretty easy to feel sorry for them. A good example is Manny Gite from "Into the Comics". He was an awkward twentysomething who lived with his aunt and desperately wanted to be a cartoonist. In an attempt to acheive this, he enters a contest for kids (in which the prize was getting to be in a comic book) disguised as his cartoon character Stoopdude and tries to sabatoge the other contestants.
Karma Houdini: Played With regarding Manfred Gite from "Into the Comics". Whereas most villains were taken away by the police in the end, Manfred is simply escorted away from the contest grounds after the Ghostwriter gang thwarts him. What kind of punishment Manfred got after that was never revealed. It's implied that if nothing else, Manfred's aunt was prepared to chew him out, but considering some of the antics he pulled were illegal, that should have landed him in jail.
In the book "Steer Clear of Haunted Hill", Alex and Leni's reactions to their loved ones (Gabby and Max respectively) having disappeared is positively heartbreaking. The disappearance of Rob isn't any better and might be worse from a Meta standpoint since this wasn't his Leeroy Jekins getting him in trouble; it was him being Too Clever by Half by refusing to buy that the crooks he ran into were actually ghosts (they weren't but that's not the point).
In the book where Alex gets kidnapped in the middle of a magic show, Gabby tearfully blames herself, in part because she and Alex had been bickering incessantly before the incident and also because she got the attention of the 2 stars who Alex witnessed arguing (they were among the main suspects and hadn't noticed Alex until Gabby yelled at him).