For some people it's important that they like the main characters. That's never been my criterion. I still remember the horror on my high school English teacher's face when I told her that my favorite character in “David Copperfield” was Uriah Heep. Her lip actually curled. But he was much more interesting than that simp Dora.
Obviously poor writing can cause any reader to give up, but sometimes even a good writer can fail to hold your interest. In the case of Susan Choi, whose “American Woman” I had enjoyed, I just got too annoyed with the main character. Choi's book includes plot elements from the real-life stories of both the Unibomber and Wen Ho Lee, the Chinese-born American scientist at Los Alamos who was accused of espionage. Ho's fictional main character, also named Lee, a Chinese-born math professor, is suspected by the FBI of being connected to the bomb which killed another professor. And in order for this suspicion to grow, Cho needs the professor to act incredibly guilty (and stupid) when being questioned by the FBI. After a while I just got too annoyed with him, and with Choi. I felt like she was manipulating her character in order to force her plot into the direction she wanted to go, and I just balked at following her.
So think back on books you've abandoned. Can you identify the reason you did it?