Literary Agent Nephele Tempest of The Knight Agency gives a great basic checklist for what will keep agents, editors–and God forbid, readers– reading your novel:
“1. A strong opening that not only captures my attention but introduces me to the world and to the characters. Who are the people, where are they, what are they doing, and why should I care? All too often I get manuscripts with openings that jump directly into the action and I have no idea whose side I’m on, or why the confrontation/argument/battle etc. is happening. In medias res is all very well and good, and appears to be all the rage these days, but I still want to know who the protagonist is pretty quickly, and what about him/her makes them the center of this story.
2. A good idea. I know this is vague, but the truth is you need some sort of driving concept behind your story. What makes this book different, and why are you the one to write it? This is just as true of fiction as of nonfiction. The idea needs to be intriguing, whatever the genre you’re writing, and it needs to be developed so that it progresses with every chapter, starting at the very beginning.
3. Pacing needs to build, with some quiet moments thrown in, but in general working toward the climax in an upward movement, like climbing stairs.
4. Strong conflict. Is this real? If I get a hint that everything hangs on a misunderstanding that could be solved with a phone call or a simple conversation, I’m done. So make sure there’s some real meat here.
5. Real characters. I mentioned this above. Watch out for protagonists who are too perfect, as well as villains who are all evil. These are stereotypes, and they’re BORING. Flawed characters are much more real and interesting, and they also get themselves into much more entertaining situations, often without you trying very hard.
6. Voice. Agents talk about this all the time, and it covers a lot of territory for me. Mostly it’s about what your narrator sounds like in my head. Vocabulary, chattiness, thoughtfulness, etc. Are they intellectual, sarcastic, uneducated but smart, somewhat slow, ethnic–and this is more about word choice than anything, so please don’t try to get elaborate about writing accents phonetically–young, old, etc.? Whatever it is, it should be distinctive to the story and the character. It should fit, there should be a reason for it, and it should be consistent.”
via Writing and Rambling – What Keeps Readers–and Agents–Reading.