I'm curious as to what series people started reading/watching that they at first thought were really good, but as time went on started disliking, or at least didn't like nearly as much as they used to. It doesn't necessarily have to be LN, could just be a regular series, TV shows, anime, or whatever. For me with Light Novels it would be:
Largest drop in quality: Rising of the Shield Hero. The early volumes are easily 8 or 9's out of ten. Everything the author tries is masterfully done, and she creates believable tension, likable/dislikeable characters, an interesting world, and subversive fight scenes that don't go quite as expected. As the series goes on, though, the characters get replaced by 2d harem members that do nothing but bicker and instantly fall in love for no real reason. The author just doesn't seem capable of ending fights without relying on Deus ex machina. The world-building is still pretty interesting, though. And by the end of where I read (volume 17) most of the volumes ended up being more like 4 out of tens.
Quickest Drop in quality: Goblin Slayer. The first two volumes were really interesting dungeon-crawling fair that I'd rate pretty solid 7/10. However, by the third volume, it's just side stories and harem antics which was way different from how the first two started. At the lowest, I'd say it was a 3/10 and I just stopped the series after volume 4.
I was thinking of writing the opposite too, the series that ended up climbing in quality the most, but If it started off that bad I wouldn't have kept going. So, I can't really think of anything that jumps in quality nearly as much as those series fell.
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