This is one of my favorite questions to ask reader friends. Do you finish every book that you start? If not, what causes you to jump ship? These days, I rarely have time for multi-hour reading sessions. If I set a book down and don't find myself picking it back up every time I get a spare minute ... it's not looking good. Usually the issue is that I don't care about what happens to the characters. I read a lot of horror (duh) - if you can't make me interested in whether the protagonist is about to have her brains eaten by zombie ghost worms, something has gone terribly wrong.
When I critique, I talk to my clients about what I call "fatal" flaws - these are issues that are likely to make a reader put the book down and not pick it back up. Fatal flows include the above, not caring about the characters, but also completely unlikable characters, meandering action-less plots, and too many long descriptive or expository passages. Most of the issues in most of the books I critique are non-fatal. Obstacles to a readers' enjoyment, yes. Serious problems that are going to lose your reader entirely, often not.
The good news is that all flaws, even fatal flaws, are fixable. The more critical the issue, the more work it's going to take to fix it, but we can do it! Talk to me today about how to keep your book off of the DNF pile.