The second most notable is the body count. Hamstergeddon being a good example, with at least one school bus being eaten, and two people being stomped on but totally not killed,or so Nick says. The writers know better.
According to the very same wiki, 70 people die in Hamstergeddon.
The computer used to evaluate the students' tests in Career Day? The POS 2000.
For those of you who aren't acronym-savvy, POS means "piece of shit".
Also in the first episode, the call letters of the radio station that can be seen while Zim does the flyover of the planet? WTFU.
The Zim wiki claims it stands for "Wake The Fuck Up."
In "Battle of the Planets," the seat Dib's sitting on while piloting the butt on Mercury is in the very center of the butt.
'"Halloween Spectacular of Spooky Doom,"' just... Halloween Spectacular of Spooky Doom. If you've read Jhonen's previous work and you have not seen Invader Zim, or have seen Jhonen's previous work... lets just say it's what you'd expect of Jhonen if you had him write for a kids show and then make a halloween episode. Which is exactly what he did in, say, Johnny the Homicidal Maniac. (speaking of him, he appears amid a mob of monsters in the episode!) It is crammed full of nightmare fuel from start to finish, broken only by snatches of nightmarish, surreal humor or GIR, so much Nightmare Fuel that even Jhonen has said he has no idea how it got aired. They even managed to get blood past the radar-when Dib manages to crawl through the portal in his head, for a brief moment his body is inside out, he hits a fence, and you can clearly see blood on it. And, you know, his organs. Very realistically and Squick-inducing too.
In 'Tak the Hideous New Girl', Tak introduces herself to her classmates with sausages as Valentines Day gifts. This elicits the quotes "Yeah! Weiners!" and "Weiners Roooock!". In the context of a Valentines Day episode, this qualifies as Getting Crap Past the Radar.
The entire idea of celebrating a love holiday by handing out meat. Think about it...
Zim reads about the creation of "artificial beavers" in the newspaper. After all these years, I finally realize it's a reference to sex toys.
Is it? In a show where they have robot bees and a gigantic alien-Santa, having artificial beavers doesn't seem too out of place.