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.
Big Lipped Alligator Moment: In that episode where a rabbit steals Jack's clothes (that episode on its own was kind of a Bizarro Episode), there's a scene where an...elephant...fairy...THING randomly flies by with this ghostly, wailing noise. Never mentioned, never explained. What's worse, at the end of that episode, right before the credits start, it flies past The Stinger AGAIN with that same creepy noise!
During the episode in the graveyard, Aku has Jack pinned, and delivers the finishing blow with Jack's own sword... only for the blade to bounce off Jack's naked chest, unable to hurt him. Aku is so shocked, his GREAT FLAMING EYEBROWS go out.
Aku trying to get children to prefer him over Jack by twisting every bedtime story he can think of. Probably the top crowner is when he portrays Little Red Riding Hood as an adorable little girl with GREAT FLAMING EYEBROWS, laser beam eyeballs, and deadly karate skills.
Fridge Brilliance: In "Aku's Fairy Tales," Aku tells the story of a dragon that comes out of the sky and is vanquished by a mighty warrior who becomes beloved by the people. "The Birth of Evil" reveals that, Aku was actually telling the story of his own arrival on Earth, only he's switched a few characters around. In his story, he's the beloved warrior; in real life he's the dragon.
Within the series, it is shown that Aku, having conquered Earth, has begun to spread his reign across the universe (the immigrant fish people in Episode II come to mind). Aku is the fragment of an Eldritch Abomination that threatened to engulf the universe in darkness; spreading his reign across the stars is effectively doing exactly what the Abomination originally intended.
While Jack is usually calm and collect, he never hesitates to draw his sword when the situation calls for it, and never regrets his decision on murdering droves of people. While every single thing he kills in the series is usually undeniably evil (except for X9), it makes sense why Jack would never be afraid that he made the wrong choice; it's shown that his sword absolutely will not harm the pure and innocent and was given to him by the gods of several different religions. Jack would never have to make a moral choice on his own because if someone can be harmed by the sword, that person must have done something evil to have tainted themselves.
In the "Birth Of Evil", there was a black piece of matter that landed on Earth. It drowned a random chicken and ruined people's houses by piercing through them. That black thing later became sentient and was named Aku. That means that Aku was doing immoral deeds before he was even conscious.
Aku released the Minions of Set to defeat Jack. What if they had succeeded? Just imagine these three unstoppable monsters roving across the world slaughtering everything in their path, and nothing can possibly stop them. If the world wasn't quite yet a completely terrible place with just Aku in it, it certainly would be with the Minions of Set roaming free.
Fridge Logic: Jack (possessed by Aku) takes on a tribe of little red newt-like monks with his sword. They're knocked around like crazy, but it's nothing like the bloodbath that should have resulted. Only if you've seen an earlier episode do you realize that the sword cannot harm the innocent, so only the monks that suffer at his fists and feet are actually being hurt.
"Funny Aneurysm" Moment: One episode had Aku having a horrible coughing fit, coughing up a bit of evil that infects Jack and slowly spreads. Mako (Aku's voice actor) died of esophageal cancer.
HSQ: Quite high. Most of the examples on the Awesome page involve HSQ.
Ho Yay: Nearly-naked Jack tickling a nearly-naked sumo wrestler.
Iron Woobie: Jack goes through hell and back to try to return to his native time, and comes out of almost all of his fights worse for wear. And yet, he trudges on.
Oh god, that elephant-fairy thing from Jack is Naked.
The Minions of Set for one gave Jack several "Oh, Crap" moments, once he realized he wasn't even close to a match for them, despite his many skills.
Nothing about the cannibalistic, starving, confused robot family with red internals?
Heck, what about the very start of the first episode? Creepy sound effects/music, creepy visuals of an eclipse and Aku's 'prison' and then the dark, menacing figure where the tree was a second ago. Then it turns around, very very slowly and its huge, unblinking eyes stare into the very depths of your soul.
Aku's extermination of the Dinosaurs, mostly the final one, who was slowly swallowed up.
No Yay - Oh God, when Aku disguises himself as a beautiful woman, he seems to actually show growing chemistry with Jack. Urgh, how far would he have kept up the ruse?
Older than You Think - The ignorant would think that Jack and the Spartans is a blatant rip-off of the more Memetic Mutation movie 300, even including many of the same sepia tone visuals. But the episode was made years before and was inspired by the 1990s graphic novel of which the infamous film was an adaptation. Never mind that the Spartans and the Battle of Thermopylae is from 480 BC, over 2000 years before Samurai Jackor300.
One Episode Wonder: Very few characters recur, so there are a few of these. X9 spends his entire episode as the protagonist, and deconstructs What Measure Is a Mook?. The Princess and the Bounty hunters were also protagonists, and they spend their episode preparing to fight Jack. Imakandi, the hunting pack who were one of the few groups who bested Jack. Also, The Guardian of a Time Portal, who is the only person to best Jack in single combat.