There are two things that always stand out to me in a story. Its characters, and theme. However, i find Characters to be the most important; to me at least. It seems that everytime i try to read a book that contains\characters i cant feel for, its exceptionally hard to read. I find that a book that has more details, and more story to the characters, makes more interesting, and more enjoyable. To me, people in books are just as real as the people around me, and its important in my own stories, that people really connect, and understand why the problem facing them IS a problem. My favorite types of characters are Anti-heroes, Ass-holes, people who do what is the right thing based on common sense. Examples being Rorschach, Harry Potter, Bean and Dink (from the book Enders Game), Montag (he did what he knew was right, and was willing to die for it), and my all time favorite character ever, Ender Wiggin.
I find theme the second reason why i read books, and enjoy them. I find that if the idea that the book leaves within you, the over all moral of the story, isnt exceptional, then i don't think as much. Thats why i hate those stupid teen drama books. They really are theme less, and really dont have anything that captures my attention. In the case of Fahrenheit 451, i found that the idea of our future was fascinating, and very eye opining.