As an author I think about this question a lot. I should hope all authors have this in mind when writing their books as readers are the writers most important ally. Books should not be written for the author, rather they should be created for their readers.
So what makes you want to pick up a book and start reading?
I suppose that question has a variety of answers, depending on your favourite genre of book. For a crime thriller its readers will want to the experience the suspense and excitement of finding out who has committed the most gruesome murders. For a comedy book its readers will want to laugh out loud all the way through. And for E.L. James's readers they are promised a certain amount of erotic desire.
Of course, you have plotlines and character development which play a crucial role in a book's readability. A book's setting may also draw in a certain amount of readers. For example a book set in Mexico may attract Mexicans or people with an interest in that particular country.
But for me as an avid chick lit reader, the character's emotional drive is always key.
I read a blog post recently about this subject and I have to say how much I agree with it: http://www.writing.ie/resources/laurence-obryan-on-writing-with-emotional-hooks/
Lawrence says you have to make an emotional connection with your readers no matter what fiction you are writing. And this statement is so true, especially with the chick lit genre. You want your readers to feel like the story could happen to them so you generate empathy which is a powerful emotion to experience.
So, what makes you keep on reading?