What book series left you frustrated with missed potential?
Example: One of my coworkers recommended the ACOTAR series to me after i said i enjoyed reading books that can boarder on trashy but still have decent romances in them. I read through the entire series and found that the longer i thought about each book, the more frustrated i became. Because to me, the series has some really solid world ideas and side characters who seem so interesting. But the whole series follows one of the least interesting characters who becomes very inconsistent in her characterization over the course of the story. I read through the books a second time just to make sure my feelings stayed the same. And, yep, they did. In fact the second read left me even more frustrated because so many good characters got short changed and shoved off page to accommodate the, "Feyra bad bitch girl boss and not all men, good guy Rhys story" and its a bummer. I even made a PowerPoint about it to explain to my one friend why it bothered me. So yeah, i want to hear about other readers' tales of finding a series that has so much potential that it just refuses to use and the frustration of knowing it could be something more. I'm always looking for other series to drive me to power piont making madness.
Edited 1) for spelling
Edit 2) a few people asked about the PowerPoint. Originally, I put it in the comments, but now there are way more comments, so I'll add it up here. As stated before, it was made mainly to make my friend laugh while I ranted. It's meant all in good fun and isn't really a detailed analysis. If you need a laugh, have a look