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.
Awesome Music: Anytime 30 Seconds To Math gets to play, expect it to be awesome.
Base Breaker: Howard. He's either a funny character, or an insufferable Jerk Ass with Skewed Priorities. Not helping with this is part of the fandom's tendency to excuse his worse moments as "just part of his character" while leaping on Randy for the slightest jerkish inclination (though this has lessened recently).
Julian, the goth boy, is a very popular character within the fandom.
Morgan, the leader of "Dancing Fish", is another very popular character.
The Tengu has a huge amount of fanart.
Theresa is ridiculously popular for the amount of screentime she's had, to the point where there are several Ninja Theresa AUs and other fan ideas with her as the protagonist floating around the fandom.
Evil Is Cool: In-universe, Randy and Howard would love to boycott McFist's products but they're just too cool for them to pass up on purchasing them.
Slips into Fridge Brilliance when one realizes they'd stick out if they stopped buying things from the most beloved man in town.
Ho Yay: The relationship between Randy and Howard runs close to this a lot.
Season two ramps it up by having an episode where Randy gets jealous of Howard hanging out with Bucky (titled "True Bromance") and where they have to take care of a bologna "baby" for science class and bicker over who gets to look after it.
Nightmare Fuel: Even if it's not focused on much in-show, the idea that typical high school upsets like breaking up, being bullied or losing friends can open you up to being turned into a mindless monster.
Some of the "destroy the ninja" plots are pretty horrifying when you remember they're aimed at a fourteen-year-old - McFist has tried to decaptitate him in multiple episodes, the Sorcerer almost crushed him (not in the figurative sense), one of Viceroy's robots tried to suffocate him by basically standing on his windpipe, he almost got vaporised in one episode (and if his scarf didn't have some sort of miraculous anti-strangulation function, that probably would have gotten him instead)...the list goes on.
To a degree, how the season 1 finale is played. Most heroes in shows like this are at least equal to the big villains in power by the time they hit a showdown like this one...but this episode explicitly shows that Randy is enormously, laughably out of his league when facing the Sorcerer. Even when he brings Howard as the Tengu into it, it takes minutes for the Sorcerer to win, and he's about to kill Howard when the First Ninja intervenes.
In "Fudge Factory", McFist almost succeeds in destroying the Ninja...via sticking his head in a high-speed industrial fan.
"Ninjception" pretty much runs on it, figuratively and literally. The Sorcerer Orb that Randy got at the start of Season 2 invades his dreams and basically puppets his body to the school so he can free the Sorcerer and return the orb. Meanwhile, Randy is trapped in continual dreams of him accidentally destroying the world/killing Howard while Howard continually eggs him to "stick the ball in a hole" (the Orb breaking in the dream is what destroys everything and starts the next one)...then it turns out that dream Howard is actually the Orb's manifestation in the dream and he basically ends up looking like he's been possessed by a demon (slit eyes, floating, horrible neck cracking).