As a story it worked. The plot arc was a sequence of events that occured as a result of the characters' choices. It was logical and fairly consistent. It was also character-based. Yes, it was a typical Hero story with all the archtypal Hero characters running through their parts, but each character had a personality and each character's actions arose logically from that character's personality. Luke was consistently rebellious and impatient in a teenager-y sort of way. Han Solo was out for his own interests, some of which were money, acclaim and love. Obi-Wan was the sage looking at the bigger picture. It all worked.
And then, the sequels piled up. As I see it, when you have a hit movie that's character-based and you want to make a sequel that ends up a flop, it's because the sequel is plot-based. In the first movie, you say "here's a group of guys that are like this, and they have this situation and let's see how it plays out." In the next movie you say "what happens if we take them and put them on a volcanic island surrounded by buxom zombie women and they're rescued by Martin Van Buren and Shaquille O'Neal who need them to negotiate a treaty to fix the world price of magnesium..." This plot may sound good on paper, and perhaps there are characters who can carry it off, but it's unlikely that if your first book was "Gone With the Wind" you're going to be able to logically shoehorn Rhett and Scarlett into that situation. In order to make the sequels move, your characters give up the internal consistency that make them compelling, and become stick figures that are now just ridiculous.
There are a few movies that have had really good sequels. Toy Story was wonderful and its sequel was even more wonderful. The characters acted in an internally consistent manner, and they were given antagonists who were also fully-realized and acted consistently. The Prime Suspect series is another example of sequels that allow the main character to be the focus, not the plot. There have been some weaker episodes of that series, but on the whole, it remains strong.
So, that's the lesson here. If you have to make a choice, go for character.
