I recently had a discussion with another author about whether a book had a true story arc. When I had read the book, it seemed to me the story arc was missing, and rather, the book was merely a series of events strung together. The story was a basis for establishing the main character's development from boyhood to manhood. While interesting, I kept waiting for the climax and resolution to the story...which, in my opinion, never came. The events in the story happened to the character, and the character never made one critical choice that brought the story to climax.
So, what's a story arc? The best explanation comes from Nigel Watt's Writing a Story and Getting Published. Nigel describes the 8 point story arc. In summary:
1. Stasis, the every day life in which the story is set. This is the setting in which the character lives, whether it be with an adoptive family, the mean halls of high school, a bored princess.
2. Trigger, which is something beyond the main character's control and which sparks the story. In a fantasy, this could be your main character falling into a portal into another world.
3. The quest stems from the trigger. If your main character fell through a portal into another world, his/her quest may be to get back home.
4. The surprise is the whole middle of the story. These could be nice surprises, but usually involves conflicts, obstacles, complications and trouble for your main character. After all, if your character found it easy to get back home after falling through the portal, that would be a pretty boring story. The writer shouldn't throw in random "surprises"...they all should be linked and make sense. Going with our falling through the portal theme, a surprise could be that after falling through the portal, the character discovers he/she plays an important role in that world's political structure. There may be forces at work that do *not* want the character to go back home.
5. Then comes the critical choice. This is where the character's true colors shine. This can't be something that happens to the character, this has to be an event that shows the character making a crucial decision. Should the character jet back home, or should the character stay and fight to save the world he/she has become a part of? The critical choice is usually the most difficult choice for the character to make. Yeah, it would be easier to go back home, but the harder choice would be to stay and fight. If your character has mettle, he/she will make the *right* critical choice.
6. So, your character makes the critical choice to stay and fight, and that brings the story to its climax. What happens after this choice is made should be dramatic, building tension so that you can't wait to see if the character wins or fails. Your character's critical choice could be facing some inherent evil overlord, someone who is stronger than your character, and maybe your character (or others close to the main character) feels he/she has little chance of winning.
7. Your main character is afraid but moves forward into battle...and then comes the reversal. The main character is stronger than anyone realized, and the evil overlord has underestimated his opponent. The evil overlord is defeated by the main character.
8. It's time to bring your story to its resolution. Your story returns to stasis, and your main character should have undergone a change and has learned through his/her ordeal. The story should be resolved (and this is important) but could also end with another trigger for a sequel. Your character won the battle with the evil overlord, but didn't kill him. The evil overlord has vowed vengence and will return.
So, not only is it vitally important to have a story arc, it's also vitally important to have resolution. Don't leave the story suspended at the end hoping to titillate your readers to wait for the sequel. That's just mean!