Sword art online. After finishing part 1 of sword art online, I thought that all the hype for it was real, than I found out there was more. If they would had simply stop after kirito woke up, than I might had been okay with the series, but instead they try to milk the hell out of it.
Deathnote. While the concept and execution was great, it started to flop after the second part. Light was a douche bag. Most of the characters were boring, even Light. I get it Light, you smart but somehow got deceived by a simple book switcheroo.