Awesome Music: The Halloween version of the SpongeBob theme song, courtesy of The Ghastly Ones.
Catharsis Factor: Seeing the townsfolk be immensely terrified upon the sight of SpongeBob's exposed brain qualifies, given how they'd been scaring the bejesus out of him throughout the entire episode, with the likes of Squidward, Mr. Krabs, and even Sandy included.
When SpongeBob goes to Squidward's house after becoming a snail, is he doing so because he wants Squidward to be his owner, or is it because SpongeBob is trying to get Squidward to help him get back to normal? Or alternatively, is there no rhyme or reason to it given that he's now an animal without developed thinking skills?
Broken Base: A majority of viewers think this episode is one of the worst pre-movie episodes, but others think it's a horror episode done right and has good jokes. There's also a lot of viewers who think it's average.
Fanon: Many fans have come to believe that "I'm Your Biggest Fanatic" serves as an Interquel to this episode, believing that the jellyfish convention in that episode takes place in Ukulele Bottom. It also helps that when SpongeBob comes home in this episode, he's wearing the same hat he wore at the start of "I'm Your Biggest Fanatic".
Jerkass Woobie: Despite Squidward's disregard of Gary, the remorse and trauma he gets after that can make him really pitiable.
Then there's Squidward's reaction when he sees SpongeBob after the fact; most of his panic is played unnervingly straight, only to end with him getting injected. The tense music ("House of Horrors", a song known for playing in tense and scary moments in the series) playing during said scene doesn’t help.
Negated at the end when Gary, SpongeBob, and Squidward meow to the tune of "Yo, Ho, Blow The Man Down," annoying Patrick. Though considering what we went through to get here, maybe this was intentional.
The scenes of the starving, parched, near-death Gary, which can easily hit home for pet owners, especially a shot where he appears to have a spiderweb under one of his eyestalks.
Squick: SpongeSnail's eye shrinking and retreating when touched...like a real snail.
Tear Jerker: When SpongeBob comes home to see that Gary is extremely sick, he bursts into tears and cradles the little snail in his arms. Definitely a case of Realism-Induced Horror for pet owners.