I got to wondering the other day, what's the most important thing to concentrate on when you're writing? I decided to look at this thing logically- go through all the big issues, and then see which matters the most.
Plot is the "what happens" in your story. It needs to go at a quick enough pace to keep the reader interested, but not be so fast they feel exhausted or confused. It has to be full of unexpected twists, but not feel contrived.
I like to think that most people are attached to other people moreso than to things or events. So characters are responsible for pulling us into a story. Characters are the ones readers identify with, love, hate. They are the ones who make us laugh and cry and want to beat up imaginary people.
The hook is probably the first thing a potential reader will hear about your book. If they ask a friend "What's that book about?" the answer will be your hook. A girl who lost her mother. A girl who's best friend dies and she stops caring about life and starts workin in a strip club. A boy who discovers he's the chosen one to lead his people out of opression. A hook is probably the reason someone picks up your book in the first place.
Theme is a big picture concept behind a book. For example while the hook for Hunger Games might be something like, "A girl enters a fight-to-the-death, in order to protect her sister," the theme(s) would be things like war and capitalism.
This is the locale where your book takes place. In contemporary books, this is a current city or town. It also includes the political and social climate, as well as the physical environment. In fantasy, this includes the rules of the world (magic, dragons, poisonous creatures), the history, the government, etc.
Unless you roll in hard-core reader circles or writer circles, you probably won't hear anything about the writing in a book before you read it. Writing needs to be accessible to the reader. Done well, it pulls a reader into the story, and makes them experience all the right emotions at the right time, and stays with them after the last paragraph.
So which of these is the most important? Well, the importance of the individual facets of a book change with the genre. For example, in YA- especially paranormal- I think characters are at the top of the pyramid, because plots can be similar and because teenager-hood is so much about identifying and fitting in. Mysteries are heavy on plot. Literary fiction boasts beautiful writing. Fantasy has intricate settings. Dystopians have lofty themes.
That doesn't mean that you can sacrifice characters in your dystopian. Or plot in your literary fiction. Or characters in your mystery. Because regardless of genre, a bad plot, paper characters, a hook that doesn't live up to it's potential, a preachy theme, a poorly-painted setting, or poor writing will still pull a reader out of a story.
So what IS the most important thing?
The most important thing is to keep your reader connected with your story. And to do that, you need to develop ALL the facets of your writing to a certain level.