First, Roger Ebert has an article on movie sequels:
“No movie executive has ever been fired for greenlighting a sequel. Once a brand has been established in the marketplace, it makes sound business sense to repeat the formula…. Movie critics despair of sequels as betraying a lack of imagination and originality. There is truth in that. But they address a hunger among fans of popular movies, who currently are waging an Internet war against Paramount for deciding not to make a sequel to 2004’s Anchorman.”
My feeling is that people are often too quick to denounce sequels as being inherently “unoriginal”. There are many cases in which they build in clever, meaningful ways upon their predecessors. It may be unusual, but it’s not unheard of.