It's always exciting to see your favorite show get a second season, but joy can quickly turn to dismay if it ends up being awful. Anime with bad second seasons are worse than series that end on unresolved cliffhangers. If a show is canceled before its time, fans can spend the rest of their lives imagining a thrilling conclusion that never was, but there's little to be done about a subpar second attempt.
Some horrible follow-ups fail to expand upon previously established lore, which usually results in a lot of asinine wheel-spinning. While the first season of Psycho-Pass presents some fascinating philosophical questions, Season 2 is basically a rehash of old ideas, so there's little there for long-time fans to enjoy. Other series, such as Rurouni Kenshin, fail because they stray too far from the original source material. Not every great show needs a follow-up, as more content leaves additional room for failure.
The anime community has mixed feelings about Sword Art Online, but most agree the first season is far superior to the second. Season 2, AKA the Fairy Dance Arc, undoes many of the good things set in place early in the series. Asuna, the show's heroine, starts off powerful enough to stand beside Kirito in battle, but in the second season, she's reduced to a helpless coma victim who needs her boyfriend to save her. To make matters worse, her consciousness exists in the virtual world, where she's being held captive by Nobuyuki Sugou, a creep who constantly feels her up despite her aggressive protests.
Additionally, the way Kirito defeats Sugou is totally ridiculous. Out of nowhere, he's granted "admin privileges," which allows him to destroy the ultra powerful Sugou with little effort. Then there's the whole "Kirito's sister is randomly in love with him" thing, which adds nothing to the show save a general sense of ickiness.
The second season of Darker Than Black isn't a hit with fans. Instead of answering some of the major questions posed by the first season, Season 2 goes off in a completely different direction. New characters dominate the screen, and most of the old favorites are either dead or nonexistent. Besides that, the first season paints Hei as a total badass, while the second season reduces him to an alcoholic who abuses his children. Not exactly a fitting end for a fan-favorite.
One major source of tension in Black Butler comes from the question of whether or not Sebastian is going to get to eat Ciel's soul. In the second season, another demon named Claude swoops in and takes Ciel's soul for himself. This is unsatisfying for viewers who were hoping to see Sebastian achieve his goal, and also for fans who wanted to see Ciel come out unscathed. Worst of all, the story doesn't even go anywhere, as Season 2 is largely composed of filler and fan service.
One of the most powerful themes in the first season of Eureka Seven is the idea that love conquers all. Renton, a human, falls in love with Eureka, a Humanoid Coralian who is tasked with observing humanity so she can help the two species live in peace. They learn to coexist with each other's differences, and since they can never be together in the world they inhabit, the season ends with them leaving together for an unspecified destination.
In the second season, the Scub Coral (the non-human version of the Coralians) are definitively evil, and Renton and Eureka lose their children. All this suffering undoes the positive message depicted in the first season, and makes it seem like the humans were right to wage war on the Scub Coral.