There's one of these every few years as the new waves of fans come in. A series that sells itself on a dark and interesting concept and maybe a meme or character design, and then everyone stops talking about it after a few years because it just wasn't very good in execution. You had Erased just a while ago, but this goes back to stuff like Death Note and earlier. They always have at least few deaths or dark plot twists, maybe a little bit of grey morality, and some kind of gimmick that could have been done well, and they end with a dead fanbase usually slightly before it actually ends.
Basically, if you hear someone say "actually, anime is very mature" in reference to a series, odds are it's one of these.