This is an important question because editors and agents are PAID to set books down. Their job is to read only long enough for the switch to flip. Then they can set it down without guilt or remorse and move on.
For me, the key difference between a book I finish and a book I set down is purely voice. I can slog through ridiculous plots and bad characters if I just love the voice. On the contrary, if I hate the voice, the most well-researched plot won't matter at all. I try hard not to invite bad voices into my head.
This voice question is always an issue. I think you can develop all kinds of skills as a writer -- you can improve your grammar, your plots, your structure. But you just can't fake a voice. I'm sorry. You can't. Don't try, because it's only painful. And, really, this is a pretty quick calculation. I can usually tell within about 5 pages if I like this author's voice, sometimes much faster. If Gravity's Rainbow wasn't so much fun on the page, I would have run screaming from the room. But it was fun, and funny, and mordant, and smart and weird. So even if I KNEW I wasn't "getting it," I kept reading it because Pynchon's wicked awesome on paper.
I've heard before that you can develop voice by mimicking other writers (not just reading them, because that's not going deep enough). I realize now that I did this a lot when I was a kid. My first stories in grade school were about an extraterrestrial dragonfly from a planet called ... Narnia. Then there were years of J.R.R. Tolkein, and then Hemingway and on and on. So did this help create a voice, or would I write the same either way? I often wonder, if you were writing in Sanskrit or Arab or Kalahari, would you have the same voice? Or is voice impossible to divorce from language?
I don't know the answers to any of these questions, but I will always believe that voice is the difference between a great writer and everyone else ...