Without hesitation, I'm going to say that voice is the most important thing you can have as a writer. Surely someone out there will disagree, and I welcome that point of view, but my opinion is that without a unique voice, your book is doomed.
Okay, doom is maybe a little melodramatic, and who am I to give advice anyway? I've got one book and it's not even out yet. But, and I hate admitting this, I suck at plotting. It's the thing I do worst. Maps, I don't need no stinking maps. But I'm pretty decent at voice. In fact, I think that my voice was what helped me land my agent and my deal.
Here's the secret about voice: you have one. It's unique and it's yours. You are the only person who can rock it.
I very distinctly remember when I realized my writing voice was crap. I'd finished a terrible YA book about Norse gods and stuff, and I was contemplating writing a follow-up book. But it was boring. The plot was involved. It had giant wolves and witches and gods and knights and pasty ogres. But it was so tedious to read. That's because the voice sucked hard.
Then I read Stephen King's ON WRITING. And one piece of advice really sunk in: Be honest.
Let me say it again: Be honest.
That really resonated with me. The next voice that popped into my head was Ollie's voice. He was a sex-obsessed teen with one day to live. He thought about God and death and breasts. To him, it was all games, girls, and food. When I sat down to write him, I promised myself I'd be totally honest. I'd be true to Ollie's character.
And it worked. Ollie was funny. Hilarious, I hope. Sure, I cringed when my mom read it, knowing she'd be reading about the secret world of a teenage boy's innermost thoughts, but I'd made a promise to be honest, and I was. Even about THAT.
I'm reading two books right now that suffer from a lack of voice. Both have fantastic plots and interesting, layered characters, but the voices in both are tedious, and that's made reading them tedious.
I think, if you want to write a good book, then you have to be honest. If your character is a sex, food, and gaming obsessed teen boy, don't try to fill his mouth with SAT words (unless, of course, he's studying for the SAT's). Don't be afraid to speak plainly and about things that might be embarrassing.
Yeah, okay, I really thought I had more to say about this, but the truth of the matter is that the best thing you can do is forget all the rules and say what you have to say as honestly and simply as you can. You have a voice. Don't be afraid to use it.