Have you ever seen an anime that you thought was going to be fantastic, but ended up being kind of terrible? Disappointing anime like this appear in every genre, and they can make you regret investing your time and excitement in them in the first place. The feeling can be especially severe when it's a heavily promoted anime that everyone is excited about before it debuts.
What are some anime that didn't live up to the hype? One of the most famous examples is Tokyo Ghoul √A, the second season of a popular show that takes the series in an unwanted direction. In a completely different genre, Glasslip takes a cool premise and turns it into utter nonsense.
This list is entirely subjective. If you see your favorite on here, it's not that it's inherently terrible - it's that there were common expectations for it that weren't met. Vote up the anime that you think could have been much better, and vote down the ones you think are fine the way they are.
The Dragon Ball franchise is so well-loved that it's hard to imagine that any part of it could be disappointing - but Dragon Ball GT was definitely that. Rather than waiting for the original manga artist to create additional material, Toei Animation prioritized milking the franchise for all it was worth by creating a non-canon continuation of Goku's adventures.
This didn't have to be a bad thing - just because a series is anime-only doesn't mean it's inherently terrible - but few elements of GT were appealing enough to allow the series to stand on its own. The protagonist, Pan, was a controversial character who some fans found deeply annoying. Even if you liked Pan, you may have found it difficult to accept Goku being transformed into a child and remaining that way for the bulk of the show. GT had some cool elements, but it was far from what most DBZ fans were hoping to see from a continuation of the franchise.
Berserk began as a beloved 1989 manga series that enjoyed a successful 90's adaptation. Because there was so much to the original manga, it's only natural that a reboot or sequel would have appeared at some point. Unfortunately, Studio GEMBA's 2016 effort left a lot of fans totally unsatisfied. The anime made liberal use of 3D animation, also known as CGI. In the right hands, CGI can enhance a series and make it look beautiful. Done incorrectly, it can completely ruin a show. The latter happened to Berserk.
Much of the terrible CGI was fixed for the Blu-Ray release, but that does nothing to improve the experiences of those who watched the TV version. What's more, there are still issues with storyboarding, editing, and other technical problems.
P.A. Works has a reputation for high-quality anime, which is part of why so many people watched the first episode of Glasslip with high hopes. The advertised storyline sounded adorable - it looked like a slice of life series about a group of friends participating in a glassblowing club, with an added supernatural element - a premonition like experience called "future fragments."
The problem with Glasslip was that it didn't do either of the things that a slice-of-life show is supposed to do. Glasslip doesn't really have a plot. It's okay for this kind of show not to have a particularly involved plot, but if it doesn't have that, it needs to have compelling characters with more than one attribute. Touko can be summed up by her catchphrase, "EHHHH?!!" while Yuki does basically nothing except run the whole time, and Sachi manipulates people. Without compelling characters or a solid plot, a slice-of-life show can succeed on its humor - but Glasslip isn't funny, its needlessly dramatic.
Also, "future fragments" aren't anything. The supernatural element leads nowhere and seems like window dressing intended to disguise a nonexistent plot.
Tokyo Ghoul √A is the second season of the widely acclaimed Tokyo Ghoul. It's also notorious for being a terrible follow-up. Why? Because it doesn't actually follow the manga storyline. Ken Kaneki, the protagonist of the series, joins Aogiri Tree, a terrorist organization who captured him, tortured him, and assaulted his friends. While it's understandable that someone might react in this way, that's not even remotely close to what Ken Kaneki does in the manga. It's not the kind of person he is.
Manga isn't inherently better than anime, but when an anime studio decides to completely change the main character's most basic personality and behavior, it isn't a good look. Changing something so fundamental means that the rest of the series has to change too, or continuity falls apart completely. While it's kind of cool to see how the story would go if things went in a wildly different direction, that's a thought exercise more appropriate to Tokyo Ghoul fanfiction, not a professional adaptation.