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.
The eclectic selection of stock sound cues have some very memorable tracks of music, particularly cues like Beethoven's "Pastoral Symphony" and "Clair De Lune", among other obscure pieces of classical music.
No wonder, since this is the melody of "God Save The Queen".
In the German dub it's absolutely beautiful. Especially when Ren harmonizes with Stimpy.
Bizarro Episode: The series is very offbeat to begin with, but even then some episodes stick out like a sore thumb, such as "Haunted House" (which was an unmade Tiny Toons episode that Spumco was supposed to have a hand in, but ended up making for themselves).
Creator's Pet: George Liquor. Many people (both in-universe and in real life) dislike him, but John Kricfalusi, somehow, is really fond of him (possibly due to him being the only Ren and Stimpy character he still owns).
A more nuanced example, since the character actually only appeared in a couple of episodes of the series, largely due to Nickelodeon's hatred of the character and thus having no problem handing Kricfalusi the rights to him when he left the series. He has a potential future however, with Kricfalusi himself as The Other Darrin.
Ear Worm: Many of the stock sound cues, including classics like "Pizzacato Playtime", "Screw on the Loose" and "Folli the Foal".
The Muddy Mudskipper TV show theme.
The LOG song.
The "Loop of Death" music from Adult Party Cartoon, which is a recording of the classic jazz song "Hold That Tiger".
Esoteric Happy Ending: Ren and Stimpy's tearful reunion in "The Big Shot" is cut short by Ren slapping Stimpy for giving away his 47 million dollar fortune.
In "Rubber Nipple Salesmen", Ren and Stimpy succeed in selling their rubber nipples to a suburbanite couple... who then kick Ren and Stimpy onto the backs of a couple of crazed bulls that they ride into the distance.
"Powdered Toast Man" ends with PDM becoming president after saving the Pope; in office, he promptly burns the Constitution to start a fireplace as he spends time with his lovely assistant.
"Man's Best Friend". By thrashing George Liquor half to death with an oar, Ren earns his approval and praise for being such a good guard dog. Ren and Stimpy are then rewarded with doggie treats, and the three of them dance happily to an upbeat Raymond Scott song. (The aforementioned thrashing was one of the things that pissed off Nickelodeon and led to them firing John later.)
The romantic tension between Powdered Toast Man (whose secret identity is that of a clergyman) and his Catholic schoolgirl assistant. After the Catholic clergy sex abuse scandals of the 2000s, this comes off as really unsettling (though the only good thing you can take from this is, "At least the lovely assistant wasn't a boy. Then it would have been unwatchably awkward.")
Freud Was Right: All over the series in regards to Ren And Stimpy's relationship. And as mentioned above, the implied romantic tension between PTM and his assistant in "Powdered Toast Man", especially the ending shot where they start a fire and roast a weenie and marshmallow over it.
Genius Bonus / Truth in Television: In "Fire Dogs", Stimpy puts out the building fire with kitty litter; in real life, Kitty Litter is usually inflammable, since it's often made out of ground up clay.
Grand Theft Auto Effect: Due to it using so many stock music cues, some of these pieces of music have become strongly associated with the series.
"HAPPY HAPPY, JOY JOY!" note Stimpy's recurring catchphrase, as well as the famous song number from "Stimpy's Invention".
Ren's infamous catchphrase "You eediot!"
The "Ren Snaps" sequence from "Stimpy's Fan Club", where Ren imagines himself as President, pressing "The Button" which atomizes Australia, has become a popular meme on YouTube called "You Dare Not Agree With X", where a picture of X character is photoshopped over Ren.
Seasonal Rot: The Games episodes. The Adult Party Cartoon episodes tried to turn things around, but, alas, those episodes made the Games ones look like the Spumco ones.
Seinfeld Is Unfunny: This is hands down one of the most ripped off cartoons of all time, basically to TV cartoons what Snow White is to animated features and what Mario Kart is to racing games. Nearly everything this show pioneered that was considered unique and groundbreaking when it first appeared has been copied, imitated, homaged, parodied, and ripped off for decades since its debut, and so many elements of it have made its way into modern cartoons that the actual show seems very mild by today's standards—much to John K's chagrin, since he felt that those cartoons only copied the many, many mistakes he made on the show, instead of what he felt he did right.
Though, unlike most cases of Seinfeld Is Unfunny, this show is still as raunchy and disgusting as it was back in the 1990s (even more so, now that the fans who watched this as kids have grown up), so much so that it has a 12 rating in the UK (and the song "The Lord Loves a Hanging" song was cut, because the BBFC objected to portraying hanging oneself as a fun activity and not a painful, excruciating death) and Common Sense Media's website put this series as "Unsuitable for Anyone Under 15."
Signature Scene: The "Happy Happy Joy Joy" sequence from "Stimpy's Invention" is by far the most iconic scene in the series.
Uncanny Valley: That female anthro chihuahua with a realistic head (whom Ren falls for) in "Marooned".
Weird Al Effect: The LOG song is very well remembered from the show, but very few people realize that besides being a parody/homage of old toy commercials, the titular song borrows its melody directly from vintage Slinky commercials.
The "Loop of Death" music from an episode of Adult Party Cartoon is actually an old jazz number called "Hold That Tiger", and the recording used is from an old Betty Boop cartoon.