The fuel that drives a story is the antagonist's goal.
Now, your main character's goal is more important. It gives the story shape and flavor. It's what the audience cares about.
But it doesn't drive the story.
The antagonist's goal provides the opposition to your character. It's what gives the story conflict, and conflict is the source of all drama. And this is true whether you're writing a literary story in which the antagonist is the meaninglessness of the universe or an action story where the villain wants to kill the protagonist.
The antagonist's goal doesn't have to be exciting or new or complicated. It just has to be solid, understandable, and believable and have enough oomph to drive the story without too many explainations. After all, the villain is not on screen as much as the hero, and often keeps his true motives hidden. We're going to spend a lot more time with the hero's goals, so those are the ones that can be most interesting and subtle and complex.
One term for what the villain wants is a "MacGuffin." Some people get the MacGuffin mixed up with "plot coupons." Plot coupons are just things your character has to seek. As in a fairytale where the hero has to seek out a magic sword and a spell book and a ring before he can acquire the magic to kill the dragon. And I can understand why people would get these mixed up -- they are similar.
Hitchcock more or less invented the term MacGuffin. He defined it as: "It's the thing that the spies are after but the audience don't care."
And it's true, the audience doesn't really care about the villain's goals any more than they care about the magic book or any item in the story. They only care how those things impact the character.
The thing that sets a MacGuffin apart from your average plot coupon is motivation. It isn't just what your main character needs to perform some task, it's the thing that drives and motivates the forces that oppose your hero. It is the secret, the true source of all the trouble. It's like the Dude's rug... it ties the whole room together. (Or for those who haven't seen The Big Lebowski: it's like the Force, which binds the universe together.)
If you want a story that really holds together and seems to drive from one end to the other, motivate your villain. (Even if that villain is "fate.") It's got to be good enough to make your villain persistent and strong. Remember that your hero is only as strong as the force he opposes.
Tomorrow an update, and then this weekend I'll post another excerpt from The Adventure of Anna the Great, which features the villain, and his horse.