Characters. I've come to the conclusion that they are one of the most important things within a book. They define the plot, the emotional connection that a reader has to the book, and the tension.
Without the characters, there is no plot, even if your characters are mearly vechials of said plot. But more importantly then that, and something I'm only just realising: Your characters are your desciption.
How do you make description matter? Well, you narrate through your character, and make it matter to them. If you want to get across the fact that your female MC is a buxom beauty, you don't say: She had an hourglass silhouette with perfect lips, blond hair and green eyes. That is a list of physical characteristics.
But if you describe the female MC through one of the other characters, it makes those details matter to the reader. Because while you're still describing physical characteristics and telling us about them, you're also showing the effect that these characteristics has on your viewpoint character. In essence, telling by showing.
For example : Christy reminded her of a whore sometimes - puffed up lips, dyed platinum hair and a chest that was too large to actually be real. But every now and then, there was a flash of something in her probably fake green eyes that made Sara hunt her down every time she wanted information.
Now, you're still getting across exactly the same details. But these details are now important - they effect the standing of Christy in Sara's eyes, and they influence how she acts around her. The have importance to the character, and because character and plot are connected, to the plot.
If Christy had been the bookish type, maybe the contrast between her appearance and the knowledge in her eyes wouldn't have jumped out as Sara quite as much. Maybe she would have gone to someone different.
And thus, you would have a different plot.
Characters determine plot, and plot determines characters. But by telling us things by showing it to us through the reaction of characters, you make the telling a lot more interesting and meaningful