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.
Base Breaker: Masato Kusaka. Many like him for being a Jerkass and others hate him for the same reason, mainly by the Western fans.
Badass Decay: Takuma. At first, he's able to curbstomp all three Renegades on his lonesome, is enough for one of them in human form, will kick your ass with poetry, and at one point wields the stolen Faiz Gear against the good guys (admittedly, though, that last seems to happen a lot.) Later in the series, he's a constantly weepy, whimpering Butt Monkey. Takuma circa #20 and Takuma circa #40 have zero in common and not in a good way. To be fair, this trope is Justified; and somewhat Foreshadowed! He IS a dangerous enemy...right up until the episode he gets his butt kicked and nearly killed by Faiz in Axel Mode. After this, at some point we see Lucky Clover gathered in front of some 'Masterpiece' paintings carrying on a discussion; which boils down to him saying "at a certain level, if it isn't perfect its garbage". One can presume that before that episode, he was so used to being on the giving end of a Curb-Stomp Battle that nearly getting killed broke him, and THEN he gets beat up by a Delta Gear user. From here it becomes clearer and clearer that he was always a Smug Snake and is simply undergoing a much slower version of such characters comeuppance.
Esoteric Happy Ending: The movie's ending; yes, Takumi saved Mari, but the world is still overrun by Orphenochs with the human race near extinction and the resistant force abandon Mari and her friends.
Hilarious in Hindsight: Remember the opening sequence? In a moment, Takumi is sitting on a chair looking like he was going to have his hair cut by Mari. Come the Showa vs Heisei movie and Takumi is there...with shorter hair.
Magnificent Bastard: Masato Kusaka. He's a scheming Jerkass of epic proportions, and the epic proportions are what make him fun to watch. He's a "goodguy" version of The Starscream (and the term "good guy" is used very loosely—he's only a "good guy" because he hates the villains only slightly more than he hates the heroes). He's popular in Japan, but the popularity could be the enjoyment of a good bad guy. He really is the series' best villain, and a rare instance of it being shown that not everyone who doesn't want the apocalypse is necessarily a sweetheart in all (or any!) other ways.
Narm: Dragon Orphenoch stopped beating up Faiz due to a toy airplane.
The Badass new Rider in Paradise Lost... who speaks English. Not Gratuitous; he speaks perfect English and, other than "Henshin," he never speaks a word of Japanese.
Nightmare Fuel: Just how do Orphnochs kill and/or turn a human? By having tentacles enter their insides and go into the heart (Though Kiba just downright stabs the heart with his sword), shown rather graphically (against a backdrop of a chest X-ray, thankfully).But Wait, There's More!. Said target then continues to live for a few minutes, wandering in some confusion before dying and turning into dust... or reviving as an Orphnoch.
Said heart bursts into flames and disintegrates into nothing
The scene showing the Orphnoch King eating an Orphnoch. Petrifying it first, then breaking its face off and eating it with crunching noises, then opens its mouth with a simple but HORRIBLY disturbing CGI effect.
It's also scary because the body of the first petrified Orphenoch, which the King didn't get to eat, was indestructible. That makes it freakier.
The Woobie: Kiba is established as this in episode 1. He wakes up after a 2 year coma and it's revealed his previous happy life is no more. His parents are dead, his girlfriend hooked up with someone else and his house is sold because his corrupt uncle needed money.
Yuka gets it just as bad. She's not only The Unfavorite and has to deal with Abusive Parents who absurdly dote on her (half? step? foster?) sister, but said sister is an Ungrateful Bitch who, after Yuka brings their parents to her when she gets hurt, lies off her ass and tells them that it's Yuka's fault in the first place. Apparently, Yuka's property is also routinely stolen and/or destroyed at school. In an early episode, a group of girls tackle her to the floor and steal her shoes and socks - the next time she sees her stuff, it's torn and cut to pieces. Is it any wonder that when she first became an Orphenoch, she took no prisoners?