Why this is a mistake: Think of your favorite book. What do you remember about it? The plot, or the characters? I would be willing to bet it’s the characters. Yet too many writers focus on the plot of their stories, emphasizing it to the detriment of their characters. When they pitch their story, the focus is on plot or situation, rather than character. But since all stories have been done, it’s going to be difficult to stand out doing this.
The solution: Focus on character. Understand that your protagonist and your antagonist (and to a lesser extent your supporting characters) are going to be what makes your story unique. The story you are going to write has indeed been done before. But your characters have not necessarily been done inside of that story. It’s that combination which will make you unique.
There is something about them that quickly engages the reader’s empathy. The great antagonists—Hannibal Lecter, for instance—also draw the reader’s emotions.
When you think about pitching your book, consider what you are leading with: Is it story or character? Optimally it should be both, but if you have to emphasize one, it should be character.