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.
Ensemble Darkhorse: Some of the monsters such as Count Burns, Devil Flanders, and King Kong Homer. An unusual example is Krusty in vampire getup from Treehouse of Horror IX who, despite not really playing a role in the segment, has been seen in Treehouse of Horror merchandise and media.
Family-Unfriendly Aesop: Ned teaches one to Bart, Lisa, Milhouse and Nelson about the seven deadly sins in Treehouse of Horror XVIII.
Hilarious in Hindsight: In one episode, Snowball 2 is brought back to life with occult. In the recent Archie Comics miniseries, Afterlife With Archie, an attempt to bring back Hot Dog starts up the zombie apocalypse.
In the Treehouse of Horror X short, 'Life's a Glitch; Then You Die', a virus (in this case, the fabled Y2K Bug) causes chaos around all around the world upon computers, and computer-controlled devices starting from Homer's computer, leading into the apocalypse. Around 10 years later, Summer Wars would also have it's plot kick off by some guy's computer device accidentally triggering a virus, and causing havoc to devices around the world. But the big difference here is that Kenji never unleashed it (he just got one letter off, and by being the first to respond, he was framed), it was Wabisuke's doing, and it doesn't end on the apocalypse.
Treehouse of Horror III to Treehouse of Horror X featured several disturbingCouch Gag sequences, including: the Simpsons family appearing as skeletonsnote Treehouse Of Horror III, also used as official promotional artwork for Treehouse Of Horror II; erupting from the ground as zombiesnote Treehouse Of Horror IV; mismatched Frankenstein-esque monsters note Treehouse Of Horror V; this one serves as a Self-Parody to another Couch Gag where normal Simpson family members have mismatched body parts, but while in that one they correct the mistakes, in this one they screw it up even more; open-eyed hanging victims (with Maggie still sucking her pacifier)note Treehouse Of Horror VI; this one comes right after a Headless Horseman Krusty throws his own head at the screen, with the resulting blood splatter spelling out the title card; The Grim Reaper sitting on the couch, and the family members dropping dead one after another (and on each other) as they rush to the couchnote Treehouse Of Horror VII; here, after this happens, the reaper leans his feet on Homer's corpse; the family getting electrocuted to death by an "electric couch"note Treehouse Of Horror VIII; the family actually never making it to the living room due to screwing up the timing of the openingnote Treehouse Of Horror IX; it could be argued that Marge and Maggie don't actually die, but even then, they would find Freddy Krueger and Jason Voorhes waiting for them.; and a Grand Finale of sorts for the couch gags with a Continuity Cavalcadenote Treehouse Of Horror X; said cavalcade consisting of Homer-in-the-box (THOH II), Alien-Maggie (THOH IX), Half-The Fly Bart and Witch-Marge (both from THOH VIII) where the Alien Maggie incinerates the Odd One Out, Lisa "as an axe victim", as soon as the latter questions what aliens have to do with Halloween.