Broken Base: The difference in format between the first and second half the series. The first half of the series is largely an episodic Monster of the Week formula, with character development and the main plot taking a backseat. While some fans feel this is a boring start that drags down the rest of the series, others feel its needed to establish the world and its themes.
Looked at from another angle, you could say that the first and second half of the series function as a season one and two respectively; there is even a natural break roughly halfway through (after the episode Time to Say Goodbye) that would function perfectly as a gap between two seasons of a show.
Complete Monster: Of all the people Robin and her companions had encountered, these one-shot characters are the most monstrous:
"Dancing in Darkness": Kazuma Kurata is a witch and Serial Killer who drains other witches to prolong his life, agonizingly reducing their bodies to dust in the process. With a body count across multiple countries and spanning decades in his quest to live forever, Kurata attempts to hack the Solomon Organization's database to gain access to more witches to devour endlessly.
"Sign of the Craft": Seiichiro Aramune masquerades as a well-meaning politician running a not-for-profit organization for rehabilitating children from abusive homes. In reality, Aramune and his cohorts use it as a front to murder the kids and sell their organs, even telling a child who calls out his atrocity that his business is the most useful the children could ever hope to be.
"Loaded Guns", which ended the show's Monster of the Week format and started its main story arc.
More accurately, it is the point when the show's primary storyline becomes the focus; the entire series is one story arc. Hunting witches took prominence in the first half of the series, but there were reminders of the arc in each episode. The Factory, Orbo, Robin's sympathy for the witches and why the STN-J is different are all front and centre even at the start.
"Raindrops" is when the show's Monster of the Week's storyline became more emotionally and dramatically investing.
Jerkass Woobie: Amon. His mother was a Seed who awakened into a Witch. This changed her drastically and ruined their relationship, making him fearful of the chance he might awaken one day. It's not helped when his former partner became a Witch and went rogue, forcing him to kill her.
They Wasted a Perfectly Good Character: Touko. She appears throughout the first half of the series as Robin's roommate, has Ship Tease with Amon, and is revealed to be Zaizen's daughter. After she's put into a coma, she wakes up only to be Put on a Bus and never referenced again.
They Wasted a Perfectly Good Plot: At the end of the series, the STNJ goes back to hunting. With the second half of the series dedicated to showing sympathy for the Witches and The Reveal of Zaizen's activities, you'd think the nature of hunts would've changed, especially as characters can no longer rely on orbo. This is left unexplained in favor of Status Quo Is God.
Robin. With the second half the series, after spending half the series hunting witches, she finds herself being hunted. To start, she has to watch her roommate get used as a hostage and put into a coma, the STNJ raided, and Amon apparently killed to save her. Then she has to deal with powerful witch hunters being sent after her, including by her mentor.
Michael. He reveals early on that he was a hacker forced to join the STNJ after hacking into their system. He's not even allowed to leave the building, since he knows too much.
