We've all been there. You're getting really into an anime, when without warning, the story wraps up with no real resolution. Or, worse, the main conflict does resolve, but the solution feels hastily thrown together and doesn't quite make sense. While some bad anime endings can be chalked up to poor writing, a lot of good shows never get to cover all the intricate details that make the manga so fascinating.
Falling in love with an anime that didn't finish is a frustrating experience, in part because reading the manga can feel totally different from watching the series. You can read the entirety of Blue Exorcist or Fruits Basket and know the whole story, but you won't get to see your favorite moments brought to life. Some anime that got cut short never had the chance to find their footing, which is an absolute travesty.
When adapting the story of a prison that holds secret death matches, the creators of the Deadman Wonderland anime only borrowed from five of the 13 manga volumes. Because of this, the anime ending is generally disliked, since it doesn't really resolve anything.
Ganta, the protagonist, hardly develops as a character, and he fails to recognize his friend Shiro's true, violent identity. The series also gives no indication as to whether the resistance movement seeking to escape the prison achieves their goal.
On the other hand, the manga manages to resolve some of these issues. If you've only seen the anime, you're missing almost 2/3 of the story.
For some reason, the people in charge of the Blue Exorcist anime adaptation decided to only pull from the first half of the 18-volume manga. It's hard to tell a good story while cutting out half of the details, and as a result, the anime is forced to totally diverge from the original narrative for the final eight episodes of the first season.
This means there's no animated version of the Terror of the Kraken Arc, the Illuminati Arc, the Exorcist Exam Arc, and many other storylines that deepen the characters and world.
While Bleach weighs in at a whopping 366 episodes, it still doesn't tell the complete story. The anime ends with the Fullbringer Arc, which amounts to a relatively anticlimactic story about spiritually-aware humans who can manipulate souls.
Meanwhile, the manga ending features an epic assassination plot, concludes the Thousand Year Blood War, reveals some eye-opening truths about Ichigo's past, and ties up many of the loose ends the anime leaves hanging.
In the final moments of the anime, Kyo's true form is revealed, and Tohru meets with Akito, the abusive Sohma patriarch, for the first time. Unfortunately, this event takes place early on in the Fruits Basket manga's 23-volume run.
Whereas the written version tells a complete story that ends with the family curse being resolved, none of this happens in the TV series. Instead, anime-only viewers get a watered-down version of an emotionally devastating story that leaves out entire character arcs. If any of the anime characters seem to exhibit confusing behavior, it's probably because their stories were cut short.