I think what cardaway said about applying the same quality to the sequel as the original can be said for ANY sequel, no matter who makes it.
If a sequel is made solely as a money-making venture instead of carrying the story further, it's going to suffer. Many of the classic Disney cartoons -- Cinderella, Snow White, etc. -- were made with a pretty definite ending, making it difficult to take the story further. I mean, how can you build a compelling story on "and they lived happily ever after"?
Movies like The Godfather and Star Wars were open-ended, allowing for more plot development. That's why those sequels were successful. Superman II did well, but Superman III and IV did not. Why? Poor plot and execution.