Fascinating characters, a great plot, original ideas, and humour. Put 'em into a pot, stir gently while bringing to the boil, But above all the characters make the story.
Cecil B. De Mille once said that the formula for making a great movie was to start with an earthquake and build up to a climax. The same can apply to a book. Ya gotta keep it moving.
I definitely love it when a book surprises me or catches me off guard.
The ability to grab the readers attention, lock on and hold it through the end. Reader's enjoy feeling involved in what their reading, feeling captivated makes folks not want to put the book down. If it's a series, creating spin-offs of various characters that were favorites in the original, might help to carry the series through while keeping reader's intrested in reading more.
The characters and what they're doing or saying.