First and foremost, the order of some events in the book are shifted and combined in order to fit the story into the time limit. Also, a focus on the relationships between the two siblings to their older counterparts goes beyond anything in the book as does the immense change that seems to overtake the character of Eustace. However, the biggest (and most obvious) change is the inclusion of some themes from the next Narnia book: The Silver Chair.
Lewis's book was a very idyllic stroll through the lands that surround Narnia, but it was too episodic to make a compelling and "edge-of-the-seat" gripping film. That's why Apted opted for the digression (which was originally met with disdain by the Lewis estate).
This move may help the somewhat flailing series to get back on its feet after a pretty miserable performance by Prince Caspian. By taking themes from The Silver Chair, a much smoother transition into the next movie is possible. This is something that has been lacking in the series thus far and just may prove to be the missing ingredient to box office supercess (my term for super-success).
If Walden Media and Twentieth Century Fox want to keep making these movies, they should keep this trend going and really focus on keeping a connection between all the books - especially because the cast of protagonists is too inconsistent for a movie series.
As far as Voyage of the Dawn Treader goes, brace yourselves for a fast-paced, action-packed thrill ride that still manages to connect with the human emotion. With great visuals, a good soundtrack, wonderful production value, and decent acting, this movie will not be a waste of your money!