The X-factor is that something that makes you special and different. This (X) is the reason that an agent or publisher should sign you, why a consumer should buy your book. Your uniqueness.
This X-factor is also something you need to discuss in your query letter. But what is it?
To figure out what makes you special you have to be well read in your genre and find what makes your work and what makes you different from the rest and what makes you sale-able?
Make a list of your best qualities focusing on what makes your writing different and then one for what makes you different; both aspects are important because you are a packaged deal. Make a list of all your best qualities that you think would appeal to an editor, agent or publisher and then use these attributes to your advantage when you market yourself and your work in your query letter.
If you have a hard time coming up with items ask your friends and your writing group what they think your best qualities are or for some adjectives they would use to describe you. They may come up with items you would not.
Good luck and happy writing!