The Inheritance Cycle by Christopher Paolini
All I read of this series was the first book and about 50 pages or so of the second before I gave up. I'll start with the most obvious fault: If Star Wars and the Lord of the Rings had a love child and were very heavy drinkers during the gestation period, this would be the result.
I get that doing a completely original idea without having drawn some kind of inspiration or influence from something else is pretty much an impossibility, but this author failed to take those ideas and make them his own. Instead of being left in any kind of suspense, about halfway through the first book I was making accurate predictions at the ~*big twists*~ that would occur in the second and third books (thank you for the spoilers, Wikipedia). Reading what I did of the series, I didn't get the feeling that I was reading an actual story so much as I felt that I was reading a very long list of some very premature ideas.
Two of the main characters, Eragon and Arya, also aren't memorable... at least, not for being anything but strong, quick, powerful, brave, beautiful, and pretty much every good thing you could ever have to say about a person. I didn't root for them, fear for their fate, sympathize with them, feel intrigued by them... anything. I felt absolutely no connection to these characters at all. They were just there, and I couldn't have cared less about them.
None of that's what made me give up on reading these books, though. Paolini put so much work into describing every single useless detail ever (seriously, these books would have been about half as long as they were if he hadn't), yet it seems like he didn't put a single thought into the actual story that was being told. What I read of the second book consisted of pages and pages and pages of description, a brief interruption that pretty much said, "Oh, and by the way, a bunch of people died," followed by more descriptions of unrelated things. That's about when I decided to return the books to the library without having finished them, and looked the spoilers up online.