Often when I'm judging a writing contest (and not infrequently when I'm reading a published book), about three pages in I'll roll my eyes and think, "Oh, come ON! That's just SILLY. I don't believe it for an instant." The published book moves straight to my library donation box, because life is too short to read bad books when there are so many good ones awaiting my attention. But when I have that reaction to a contest entry, I have to keep reading--and to keep all my comments and suggestions kind and tactful, because somewhere out there there's a real writer who loves that story as much as I love any of mine.
Sometimes my inability to suspend disbelief springs from a big factual/historical error. In that case, my job as a judge is easy. I point out the issue, striving for tact no matter how obvious whatever bit of reality the writer botched seems to me. (Because I am human, this sometimes requires pacing around the house grumbling about idiots who don't READ, and doesn't anyone do RESEARCH anymore, and why do people who don't care about history attempt HISTORICAL fiction. But I don't let myself comment on the entry until I'm past Outraged Historian mode.) I then try to offer a suggestion that would fix the error without drastically altering plot or character.
More often what throws me out of the story is that everything is just TOO something. Too big, too serious, too goofy, too black-and-white. In other words, over-the-top. I usually suggest striving for greater subtlety and working in some moral gray areas in both characters and situations.
But I always feel weird giving that advice, for two reasons. One is that books that strike me as over-the-top and ridiculous do get published, so maybe I'm telling a writer to fix something that ain't broke. I do try to set my personal tastes aside when judging, but it's not easy, and sometimes it's hard to tell the difference between "not to my taste" and "bad."
The other reason is that on some levels genre/popular fiction is supposed to be sweeping and over-the-top. Fantasy protagonists save the kingdom, or even the whole world, often over and over again. There's a strong thread of historical fiction where the hero just happens to meet all the important people and play a crucial role in every critical battle of his era. Romance heroes and heroines love on a grand scale. And I'm fine with all that. Phedre no Delaunay of Jacqueline Carey's Kushiel books can save Terre d'Ange a dozen times, and I'd be happy to read each new adventure. I love seeing the Peninsular War through Richard Sharpe's eyes and WWII through the Henry family of Wouk's Winds of War/War and Remembrance. All those books are GOOD over-the-top, IMO. And I think there truly is a qualitative difference between those books and the ones I want to throw at the wall--an art to the writing, a certain subtlety in the midst of the epic sweep, a certain humanity in the protagonists allowing me to identify with them no matter how much braver, smarter, or magically gifted by the gods with world-saving powers they are than I could ever hope to be.
I know the difference is there. But I don't know how to define that difference in any way that's helpful to some struggling writer who's entered a contest just hoping for some useful feedback to help her improve her book.