Anime/Manga Anime that started off good but ended terribly

[Somewhere_]

Not an anime, but I finished the manga Cage of Eden today (last chapter is 185). I really enjoyed it because of its intense action and mystery. The characters were awesome. Death kept the series tense. The monsters and enemies were interesting and unique.

But the ending was extremely rushed because the series the was cancelled. There were some subplots and characters that needed to be finished, and a good amount of questions that needed to answering.

The rushed ending really killed it to be honest.

 

[Satoshi Ookami]

It's an older one, but who remembers Neon Genesis Evangelion?

More like who doesn't? :P
But I wouldn't consider that to be something that ended terribly, it was a mindf*ck, but it wasn't bad =D

 

[jasonresno]

Man, Deadman Wonderland fits the bill for me. It started out so dark and horrible (in a good way) but quickly degenerated into cliche.