Die of Curiosity, meet the Devil in the Details
All books and movies are based on simple concepts, and thus can be summarized in one or two sentences. In fact all books and movies have such a summary on their back cover. Once summarized they can usually be put in one or two categories. Take for example the category of 'revenge movies', the summary will probably be "Main character is wronged, he gets revenge on the perpetrators'. Adding a twist at the end will make
a formulaic movie or book much more satisfying. However how do you keep a viewer or reader interested till the end? Curiosity. And how do you make your work stand out from the rest? Details.
A good way to keep the reader's interest in a book is not to let the reader know the category of the story, so he won't be able to guess what's going to happen. This can be done by either making your story's category an unusual one, or have it fit multiple categories. Having one or more twists throughout the story is a great way of keeping interest high.
Having realistic and engaging characters is a must. Describe your characters' actions and thoughts in such detail that the reader feels like he knows them, but without making them predictable. Usually the main character is a person who your readers can identify with, or who defies the odds. However a hero with character flaws is interesting too, and increases realism, since nobody is all good or all bad.
Having interesting adversaries or foes is important too. Just as having a good guy with flaws adds realism, so does having a foe with a human side, or with good but misguided intentions. Sometimes I find myself watching a movie with bland heroes, but because the evil character is so vile, I just have to watch it till the end to see justice being done. I do think the bad guys are more important to a story than the heroes.
Minor characters should be introduced with a little background to them, or maybe by describing their thoughts and how they relate to the current scene.
Describe a place and time period in such detail that the reader can close his eyes and imagine he is there with the characters. Setting the mood, describing the environment, climate, historic or futuristic details, and cultural traits are all important. If the story is based in an exotic location, or a different period of time, even describing day to day actions can be interesting.