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.
Badass Decay: Frank has arguably become this in season 3 where instead of being a bully, he's turned into a prankster and the other kids at school have gone from scared to annoyed, or in Erica's case, angered.
Crazy Awesome: How to describe Bobby Wasabi? Ninja security, disco bathroom, sausage toss...
Also, Eddie's "Everybody was Ninja Fighting" song in "Wedding Crashers".
Designated Villain: Erika from Jack Stands Alone. She may have stolen a live animal belonging to the school and had Frank take the blame, but it's been mentioned that Frank sabotages all the school dance events she plans, yet Frank is never shown getting punished, which justifies her actions to some extent. This also technically makes Jack a Designated Hero since he's defending a bully who was innocent for this one prank but guilty in several others.
Fridge Horror: Judging from what we saw of Kai in the Jack-Kai fight in "Kickin' It with China", and knowing that in the past Kai beat Jack so badly that he quit karate until moving to Seaford, how bad can Kai get?
Sensei Ty has crossed it in the past, continuously blaming Rudy for causing him to fall even though it was the Grandmaster who tooted.
Sensei Ty crossed it in the first episode, by ordering Frank to break Jack's leg.
In Wrath of Swan, Frank crosses it when he has Brody bring Kim to Swan Cotillion, and then fires gravy on to Jack and Kim
Sensei Ty crosses the line again in Kickin It on Our Own, by being brutal to the Wasabi Warriors, taking away Jack and Kim's black belts (he obviously had no right to do that), and forcing Eddie and Milton to drink heavy amounts of water to fight for their right to use the rest room. Most of this, just to make them quit.
Nightmare Fuel: During Jack and Kai's fight in "Kickin' It with China", there's a moment where Kai seizes and punches Jack's broken wrist. That's likely to give many viewers massive shivers.
The Monster Clown that pulls off Jack's blindfold and paralyzes him with fear in "Boo Gi Nights". Even if he wasn't deathly caulrophobic, it would be easy to relate to how scared he was.
Periphery Demographic: There's a large female fan base watching the show for Leo Howard. And, while boys are the intended target audience, some are just watching for Olivia Holt.
There's also a sizable female audience watching for Olivia Holt as well (no, not for the usual expected reason, but because they feel Olivia actually does bring a "girl empowerment" element to the show).
Special Effect Failure: The Literal Cliff Hanger scene from "Dojo Day Care", with Jack holding onto a construction bar with one hand and the principal's baby son with the other. Not only is it very obviously on green screen, but in select shots you can clearly see Jack holding onto a doll that looks like the baby son.
Tear Jerker: A lot of people have been upset with Eddie's disappearance in Season 3. The reaction to Kim's departure was even worse.
What an Idiot: Ironic, since Milton's the smart one, but in "Karate Games", Jerry spots a guy with orange pants and Milton just thinks the man is brave to wear orange after Labor Day. When Jerry and Eddie tell him he's an escaped convict, Milton says he's going to "subtly tell the police helicopter of the convict's location". Yet, his idea of "subtle" is to stand on a chair, yell into the sky which ends up warning the convict to try to run away.