Disembodied Eyebrows: The default style of the characters as most of them have rather large eyes and not enough forehead space for eyebrows.
Does Not Know His Own Strength: Billy has turned a part of Spencer's room into a chasm at one point during a diva fit, and has even possessed other unintentionally. Due to leaving his ectoplasm around he has accidentally created various monsters that he and Spencer in turn have to deal with before they hurt someone.
Never Say "Die": Despite the show being about a boy and his ghost, the word 'die' is never said, and the concept of death is usually dodged around.
Nice Girl: Mallory; she's so nice and so loved by everyone that upsetting her is considered a heinous crime in universe.
Non-Idle Rich: Mr. Minotaur works as an educator out of devotion for his idol Manuel P. Goppers and claims he doesn't need the pay because one of his ancestors invented the receipt.
Our Ghosts Are Different: Billy's ectoplasm can apparently bring anything to life or turn it into a ghost if consumed. Also, he can't be seen unless someone is wearing a personal item.
It's implied that this is partially due to him being bad at being a ghost, as in the series more powerful ghosts are able to appear without the need for people to be in posession of their personal items.
One episode reveals that seagull poop makes him visible. It's never explained how or why though.
Royalties Heir: When Mr. Minotaur becomes the Principal of Beverly Beverly High School and Alpha Bitch Lolo Calorie tries to use her father's wealth to intimidate him, he says he doesn't need bribes because one of his ancestors invented the receipt.
Running Gag: The "CALIENTE" joke on "School of Rockers"
Scenery Porn: The backgrounds scenes in the series are really colorful and gorgeous
Shout-Out: There are various shout outs to Mario and Tron in Episode 8, and Billy will reference other, actual bands in casual conversation from time to time.
In episode 29 the song Billy sings has lyrics reminiscent of a song sang by an irl dead popstar.
Episode 48 is basically a G-rated version of The Hangover, right down to its name—"The Sleepover".
Shrine to Self: Billy's entire mansion seems to be one big shrine to himself.
Sitcom Arch-Nemesis: Principal Ponzey against Spencer. Lolo is becoming slowly in one to Spencer, as well.
Sweetheart Sipping: Sam Hoover and Billy in episode 13 during Hoover's memory montage of their day.
Talking to Himself: Up to Eleven in Polish dub, where every person in the series who wasn't Spencer Wright or Billy Joe Cobra (who were voiced in Polish version by Karol Jankiewicz and Karol Osentowski, respectively) was voiced only by either Bożena Furczyk or Waldemar Barwiński.