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 farmers playing hideous instruments made out of piping and dead rats.
"Today...we saw Richard Madeleyyy...beating up a cleaneeerrr...and later being apprehended...for fucking a coffee machiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiinnnnnnnnnnnnnneeeeeeeee..."
The noise of a man screaming while jumping into a woodchipper is something you'd probably like to forget.
The ambient, unnerving background music in most of the sketches, including the abovementioned woodchipper sketch, which incorporates a tense, somewhat catchy loop of the intro of Screen Kiss by Thomas Dolby. Particularly creepy combined with Mark Heap's unhinged monologue and the screaming, that sketch in particular has a way of staying in your head.
The sketch where a semi-naked man and a woman run through a forest around police officers singing "Mah Na Mah Na", while a dead body is found. They then proceed to sit on the body to squeeze out a noise into a bass clarinet in time with the music. Yeah...
Not only is the source material, editing, soundtrack and general ambience extremely tripped out and unsettling, there's also the video remix version "Jaaaam" which is the same footage, but with each scene put through various visual filters - pixelation, solarisation, all sorts. It's extremely hard to watch in a darkened room and still feel sober.
The unrelated novel:
Tear Jerker: The pointless death of Angela right in front of Travis.
Travis: ...I couldn't find a stick...
The Woobie: Travis. He has a naturally submissive personality, and Tim and Don both abuse him for it. And everybody around him keeps either dying or trying to murder him.
Jerkass Woobie: X and Y. They didn't mean to cause the jampocalypse after all.