Arc Fatigue: The fact that the show takes two full seasons to identify Rosie's killer really aggravated a lot of viewers, so much so that the ratings dropped sharply for season two and never recovered.
Oh so much. Especially once it's revealed who killed Rosie. The actor's performance throughout the show until that point was certainly not what you'd expect from someone who knew they were responsible. Thankfully averted by the reveal of the Pied Piper, which had hints and foreshadowing leading to it, and it cast a significant part of the show into a new light.
Several of the clues also get this effect after a while, as it turns out Rosie packed about a week's worth of adventures into her final 24 hours.
A lot of complaints about the first season finale came down to one of these where it was revealed Holder delivered faked evidence that fingered Richmond as the killer. The second season made it clear that Holder wasn't aware it was fake.
Broken Base: The Netflix revival season has received a very divided reception, with viewers either finding the new storytelling direction refreshing and compelling, or boring, repetitive and unsatisfying. The fact that much of the drama hinges on Linden and Holder covering up Skinner's murder, which takes up about as much time as the season's murder case in contrast to previous seasons, has quite a bit to do with it.
The Pied Piper, real name James Skinner, poses as an innocuous family man while moonlighting as a Serial Rapist and Serial Killer of dozens upon dozens of young girls. When spotted in the act by a young boy, the Piper makes plans to kill him but ends up murdering the child's mother and framing his father for the deed. Killing Bullet for discovering his identity, the Piper is eventually cornered by Sarah Linden and tries to goad her into murdering him by boasting the girls he killed were "garbage" undeserving of being seen as human beings. With his claim of not harming children shattered by his vast number of young female victims, the Piper is a hypocritical, misogynistic psychopath with an ego so large none of his deplorable crimes elicit any measure of human emotion from him.
"That You Fear the Most", "Seventeen", & "Head Shots": Goldie, though a minor villain, establishes his pronounced depravity and wickedness by beating and raping the teenage Bullet for breaking into his apartment. Caught by the cops, it is discovered Goldie is both a distributor of child pornography and a Serial Rapist of underage girls himself, before he manages to humiliate the police and weasel his way out of the punishment he deserves.
Fandom Rivalry: The Breaking Bad fans weren't happy with this show stealing its attention, especially when it looked like Breaking would get a truncated final season in favor of The Killing getting a bigger budget from AMC (luckily, this didn't happen).
Growing the Beard: The third season, which focuses on a new case altogether with a new supporting cast, has been significantly better-received than its predecessors.
Moment of Awesome: "I'm not Stan." Said by Alexi right before assassinating local mob boss Janek Kovarsky. Alexi spent his childhood plotting revenge against Stan for killing his father, but after befriending Stan's lovely daughter Rosie, he decides not to go through with it. Thus, when Alexi finally learns that Kovarsky forced Stan to kill his father, his vengeance is swiftly delivered.
The Pied Piper (James Skinner) crossed it when he snapped and raped then murdered a teenage girl. Afterwards, he never looked back and kept doing to dozens more over a number of years.
Goldie crosses it when he attacks and rapes Bullet.
Holder himself felt he crossed this line when he stole from his own kid nephew to pay for his drug addiction.
The murders of the Pied Piper in season 3; the victims are raped and then have their throats cut ear to ear with a serrated blade, so deeply several of the victims are almost decapitated.
Win Back the Crowd: The third season addressed and improved many of the issues that plagued the latter half of the Rosie Larsen arc, resulting in tighter plotting, less Padding and a more satisfying and less nonsensical Reveal. Unfortunately, the bump in quality was not enough to save the show from cancellation.
The unnamed parking garage attendant who becomes a Spanner in the Works for Nikki has a lot of fans due to his friendly demeanor and nuanced characterization for an African-American character in the 1950s.
Hilarious in Hindsight: A heavy-voiced narrator describing the adventures of "Mr. Unger" may make some people think of this.
Moral Event Horizon: Officer Keenan is always the meanest of the gang, but he becomes a lot harder to care about after he apathetically drives away to participate in the heist rather than stop a man who's trying to kill someone.
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