Character interaction

Think in real life; relationships is what it's all about. Your wife, your annoying brother, your slightly stupid best friend, your antagonistic coworker. The driving force of your life, right?

There are two types of stories (well, a lot more than two, actually): story driven and character driven. With story driven, the characters are just there to further the plot. In character driven, the plot's just there to give your characters something to do. It's your job to get something in between.

Think of Lord of the Rings. Merry, Pippin, and Gimli are comic relief. Boromir, Aragorn, and Legolas are the "straight guys", or the guys that keep them in line. Gandalf is the benevolent and powerful leader, who acts as a role model to the Hobbits. Sam and Frodo are best friends, Legolas and Gimli develop an unlikely friendship, Boromir and Aragorn have a rival thing going, Merry and Pippin join the Ents, etc.

Character sub-plots and character development add depth to the story, and character development is best done with other characters. A joker and a monk learn to become friends, the monk lightening up a little and the joker learning that joking doesn't have to be done all the time.

Put a lot of thought into this; who hasn't heard of Sherlock Holmes and Watson, Batman and Robin, Luke Skywalker and Obi-wan Kenobi? Make compelling characters, and compelling interactions between the characters, and the reader will stick around to see what'll happen.