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.
The second season of Death Note has its good points. The opening and ending themes are amazing, and there are some genuinely fascinating moments involving Light's descent into madness. However, for some fans, the second season fails to reach the heights achieved by the first. The new antagonists Mello and Near aren't as richly developed as L is in Season 1, and the whole thing feels tremendously rushed.
While Season 2 does manage to stay faithful to some aspects of the original manga, it compresses and cuts a ton of vital plot points, and the resulting story doesn't always make sense. The series is about elaborate schemes and mental warfare, so viewers need to be able to follow along, yet without reading the manga, it's hard to know what's really going on.