@Lanny Most of the time I'm able to finish the series, because I get into the stories, but some of them I got really sick of, just because the writing style of the author started to bug me, usually because I notice that they constantly, redundantly describe something the same way, or I get sick of their preferred style of metaphor. Some examples are the Codex Alera, by Jim Butcher, or the Inheritance series by Christopher Paolini. I tend to be a marathon reader if all the books are out, so maybe It's a side effect of reading too much of something in a short time.
R. A. Salvatore used to be my favorite author, but in his case, I think I just grew out of his books. I've decided that most of his characters tend to be pretty one-dimensional, and I can't just read another extremely detailed, yet fairly generic fight. This is a case where I haven't actually been able to finish a series because I just don't even care what happens in the story anymore.