I'll start, Deadman wonderland, I remember watching it when I was just a teen, it was wonderfully gory, had sex appeal, an interesting premise, all the things a young guy likes. But I got to I think the third episode and I was like "what happened?" The action totally fell off, no character development was happening, it was just boring.

This sticks out to me since I have such a high suspension of disbelief so I like pretty much all media, the fact that this made teenage me stop watching a series midway just shows how much I didn't like it compared to the start.

What about y'all, what series let you down?

Shield Hero. I loved the initial premise of an anti-hero doing what he could to just survive in a world that was against him. His only companion a slave that he was raising almost like a surrogate daughter, the two of them against the world.

I thought it was an interesting premise. Then the tone completely shifted and it turned into a generic power fantasy isekai where basically everyone loved him. Completely lost me the second they introduced Filo or whatever her name was.