What is it?
Something vague - maybe clever - possibly inspiring.
Writing isn't just about a story and characters that won't go away (mine wake me up every morning with a fully formed scene in my head - it drives me nuts because I can't write every morning).
Writing - really good writing - is about the High Concept. What is the purpose, what is the goal. What do your characters really want and what makes them tick. A high concept means you have to do some homework. You have to understand you characters motivations - truly understand them - so that what happens and how the story evolves is directly related to a cause and affect that takes you to an amazing place.
It's a story that makes the reader feel both clever and empowered (because you let them in on some secrets "on purpose") and also like they're just along for the ride (because they didn't see "that" coming).
I'm working on a High Concept for my Paranotte series - and its HARD. If I want the writing to be really good, I need to watch out for every word and make sure it conveys the speed and intensity required at each point. I also need to be clever with foreshadowing, which doesn't work properly until you absolutely know where you're going.
Writing...? It's not a walk in the park - gotta love the challenge