|Spock forgot the Triscuits. Again.|
Sometimes, to buck this trend, a sequel comes along that equals (if not exceeds) the first in the franchise: Star Trek II: The Wrath of Khan, The Empire Strikes Back, The Godfather Part II, and Harry Potter and the Prisoner of Azkaban, to name a few.
Here's the basic danger inherent to sequels: if your protagonist learns something to enable him/herself to overcome their flaw and become a better person in the first film, then where is there for them to go (emotionally) in the second (and subsequent) films?
This is done deftly in the original Star Wars trilogy:
1. In A New Hope, Luke goes from technology-trusting farm boy to self-trusting Force adept.
2. In The Empire Strikes Back, Luke progresses from Force adept to a deeper understanding, choosing whether he will repeat his father's fall.
3. In Return of the Jedi, Luke the Jedi Knight becomes Luke the Jedi Master, casting aside his weapon when Darth Vader is at his mercy. He demonstrates compassion, the final lesson.
I posit that it's the very humanity that the writer injects into the character in the first film that makes it so difficult to pull off a sequel that can justify its own existence. If you tell a complete story the first time around, why is there a need for a second, third, fourth, or fifth story? And how do you know that you've told a complete story?
It comes down to stakes:
In The Matrix, Neo proves, by the end, that he can save humanity. Therefore, in The Matrix: Reloaded and The Matrix: Revolutions when the stakes are saving the city of Zion, well, if we already know he can save everyone, then saving a city really isn't all that exciting.
In Star Trek (the Abrams reboot), Nero (the villain) can travel through time and implode entire planets. In its sequel, Into Darkness, John Harrison can... punch really, really hard. As the budget rises, the stakes simply don't, and excitement drops.
Contrast this with Star Wars, The Dark Knight, and The Lord of the Rings. There's an escalation: the idea, present before the first film even goes into production, of scope. All of the cards aren't shown in film one. Rather, the stakes rise for the hero and for whom the hero protects. The more protecting the hero does, ironically, the more danger the world is in. That's an ironic arc, and that's good writing. The more Luke learns about himself, the greater the fallout from any fall, on his part. His father Anakin was a powerful Jedi before he was a powerful Sith.
Let me be clear that I don't take issue with reboots. Tim Burton did it with the Batman franchise in 1989, as did Christopher Nolan in 2005. J. J. Abrams did it with Star Trek in 2009. Superman has been through several incarnations. Beloved characters ought not to be shelved forever. However, in each of these cases, although the central character's archetypal role (hero, captain, courageous leader, etc.) always has certain elements in common with his/her predecessor(s) (or risk being unrecognizable to the audience/rabid fan base), the character him/herself and the world he/she inhabits is always different. The Gotham City of Adam West's Batman is strikingly different from the Gotham of Michael Keaton or Christian Bale. The Batmans (Batmen?) that each actor portrayed were also strikingly different, despite the character's ever-present dedication to protecting the city.
I tend to value reboots over tired sequels, but over both I value the introduction of completely new franchises, altogether. The Matrix, The Hangover, Harold and Kumar, and Toy Story are modern examples of clever, original ideas. True, these franchises each have multiple sequels (of arguable quality), but the point is that they began as risks, as stories that hadn't been told before.
I've seen Iron Man 3 and Star Trek: Into Darkness in recent weeks. Both films had spectacular visuals. Both had decent examples of solid acting. One of them, yes, had lots and lots of lens flare. But both paled in comparison to their predecessors, and whether or not you agree, I think we can concur that whatever kind of story you like to be told, the best ones tell us more about how a character handles his/her biggest challenge rather than what that character does after he/she overcomes it.