Javert: "It is my belief that we could be facing a loss of all rationality, sense of right and wrong, as well as sensible narrative - we may fall prey to some form of mass hypnosis, leading to complete moral abandon & sexual debauchery."
Arson, Murder, and Jaywalking: On page 98, Rooftop CSI, "...by letting you away with kidnap, blackmail and loud music after 10PM..." (Javert doesn't actually list by name the last straw after this, but it's definitely falls in the Felony Misdemeanor category.)
Beyond the Impossible: The main reason Chihiro is the unofficial president of the Wibsy (Weirdo Boyfrend & Supernatural Stalker club) is that, using Christine's words, "It's hard to trump dating a river." While the others have at least human-shaped (a fae, a ghost and a vampire, amongst others; Christine with her murderous deformed psycho is the most normal of them all) boyfriends/stalkers, this is impossible by their standards. In her defense, the river was in the form of a human boy that looked her age at the time she fell for him.
Blue and Orange Morality: As a Fae, Jareth is technically outside of the standard morality spectrum, which is why Aziraphale and Crowley can't really do anything about his shenanigans when Sarah complains. Observing him with Heaven/Hell's tech only results in the ++CUCUMBER ERROR++ as well.
Sarah, for whatever reason, has a video copy of her movie.
Christine has mentioned the largely fan-reviled proposed sequel to ALW's musical.
Brick Joke: Back in comic #35, Sarah wrangles a promise from Erik to wake her up if she's ever found wandering towards a certain person's room in her sleep. 2 years and 93 comics later, she gets a rather rude awakening.
Clucking Funny: As a Labyrinth fanfic random chickens appear naturally and also In-universe the characters found a lot of humor in Jareth's chicken phobia and the Chicken Coup that caused it. But as it turned out there was a goodreason for this. Yes. The chickens of the Labyrinth plot to take over the kingdom and maybe even the world.
Cuteness Proximity: Jareth uses his owl form as a means to get close to Sarah on occasion; when she doesn't react to the sad fluffy owl on her windowsill, he knows he's screwed up big time.
Cut His Heart Out with a Spoon: Here Jareth threatens Erik with magicking all the songs of Miley Cyrus into Christine's head if he doesn't let Sarah go. Later, he uses a rather elaborate threat about how he'll stop Webber before he makes the musical (leaving Erik with only his literary characterization). There are also frequent threats of bogging (being thrown "headfirst into the bog of eternal stench") from Jareth and Sarah.
Did You Just Scam Cthulhu?: Sarah made Jadis disappear by introducing her to Darkness so she manipulated two supernatural beings to get what she wants... it didn't work the way she intended but still worked.
Distracted by the Sexy: Christine when Beowulf appeared. Jareth several times ("Since when can you not tell blond from brunette, Jareth...?" "Ohh, silver lining.")
Finagle's Law: The Apartment has a fail field surrounding it, so anything that should be awesome... isn't. Like when Edward Cullen happened upon the Barbecue for Toby's birthday... (In trope terms: "The perversity of nature" is good friends with the Rule of Funny but has some serious grudge against the Rule of Drama and the Rule Of Cool).
Flat Joy: Sir. Dydimus' "Huzzah." on Ambrosius bringing back the talking hat.
I Have Boobs, You Must Obey!: One of Christine's favorite persuasion techniques is the cleavage squeeze. She also frequently combines it with Puppy-Dog Eyes for maximum effect. (As Sarah pointed it out this only works on guys.)
"[Aziraphael and Crowley are] having a long and convoluted debate about whether wiping his memories of today would be a good or a bad thing. If they come to the conclusion that it's a kindness, Azi will take care of it. If they decide it's wrong and deceitful, Crowley will."
The next time Raoul is subjected to this, Christine only comments on the potential health risks a second mind wipe may or may not cause.
May-December Romance: Played with as the characters lean on the Fourth Wall. Jareth is a fairy tale kind from... a long time ago. Sarah is in college... and was 15 in 1986, and had at least one date with Jacob Black, definitely a teenager.
Medium Awareness: Outside of the occasional bout of fourth wall breakage, the characters are seemingly aware they're in a humor comic, and will sometimes comment on it.
Mood Whiplash: Not quite as pronounced as Roommates, but it happens. See for example 211, which begins with a pretty intense discussion on why A Match Made In Stockholm really isn't a viable option and ends with Nanny Ogg smacking Jareth on the seat of his criminally tight pants.
The main difference is: GND uses this trope to ease drama with humor, while Roommates can whiplash in any way possible.
Never Wake Up A Sleepwalker: Erik advises Sarah not to wake Christine when she finds her sleepwalking toward his room for some late-night music lessons. Sarah dryly informs him that modern science has disproved that notion, then makes him promise to forget the potential trauma and just dump a bucket of cold water on her if he ever finds her sleepwalking toward Jareth's room. See Brick Joke for the follow-up.
Overprotective Uncle: Sarah's uncle, Agent Phil Coulson, who assures her that he can arrange to have any boy who bothers her sent to Guantanamo (or a "magic-proofed pocket parallel dimension," if necessary) and makes a note to get her a set of tasers like Natasha uses. Pika even describes him as being "made of overprotective awesome to Sarah in contrast to her somewhat absent-minded, harried father."
Psychic Link: Jareth and Sarah share a mild mental connection made stronger in the Dreaming. Sarah is shown able to sense a simple wedding-fantasy, and something more complex (like say a would-be-sex dream) overrides her higher thought processes. Word Of God says this goes both ways.
Screaming Warrior: Sarah's battle style involves shouting random stuff while kicking ass. Mostly things that could insult / infuriate her opposition, like the names of chicken related food when fighting the dreaded chickin' revolutionaries here.
Take A Third Option: The 200th page speshul, quite literally. In the poll for this speshul The Box and The Prom Pictures options won, but the characters didn't want either... so they so counted on the readers being distracted by fanservice. (Which "accidentally" was the 3rd most chosen poll option.)
Tangled Family Tree: Jareth's inexplicable relation to pretty much every magical blond in fiction (Jadis is his mom, Nuada is his nephew, Howl is his great great great great grand-nephew once removed...) is something of a Running Gag. This comes from Roommates. In the Buildingverse every magical being is related somehow, not just the blonds.
Tempting Fate: Sarah actually invokes the trope so it'll be subverted when she decides to rummage 'round Jareth's room, since doing so makes it predictable and thus not funny. It doesn't work.
Time Skip: Magically from the inside, even if just a little. Jareth thought that pages 198-199 had too little fanservice, so we can very well skip right to 200. (Yes. This means that the comic doesn't have pages 198-199, and even #197 turns into an error messagenote 19seeeev777.81191WTFfff++CUCUMBER ERROR++.)
Weirdness Censor: The official answer to why the muggles don't notice all the weirdness is because Aziraphale & Crowley keep up a SEPnote "someone else's problem" field around the building. Which apparently doesn't stop some muggles to date or run away from the resident weirdness.
Where The Hell Is Springfield: Used to Hand Wave the issue of time differences during the Paris arc; since nobody is entirely sure where the Building and its environs actually are in relation to France, no sense in worrying they might be calling during the middle of the night there....
Why Did It Have to Be Snakes?: Jareth is scared of black chikins. This is apparently the result of an attempted coup made by the smarter ones in his kingdom. He doesn't appreciate the humor the rest of the cast finds in it.