4. New Characters End Up Being Inconsequential
Though I've touched on this, this really deserves a mention on its own, because....well, seriously, how can you buy into someone you know you simply can't get attached to? In theoretical defense of the Star Wars Prequels they did have three films to work with and make a character's inevitable demise mean something. That said, well....let's leave Natalie Portman and Hayden Christensen alone, and instead look at a character that COULD'VE had any possible ending: Samuel L. Jackson's Mace Windu. He didn't have to die. He could've gone into hiding. Or, he could've been killed in the second film, and communed with Yoda via the force. He could've been wounded and advised Obi-Wan to accomplish things on the sly, or suffer incapacitation as he had. All of these would've, y'know, better furthered the plot too. Goodness knows those films needed devices like that. But simply, you know he's not around in Episode IV. No one mentions him, and the Jedi are all but "extinct". So, do you believe that he's the superior Jedi Master he's supposed to be? Clearly, the fact he's going to get wasted doesn't help that end. And it's hard to care for ANYONE you know is going to get sidelined or mowed down in advance of the story you love. Worse, Windu could've been ANYBODY. That rather foreheaded Jedi (Who Google tells me is Ki-Adi Mundi) on the council basically said the same nonsense, fought the same fights, and while he didn't get his ass handed to him personally by the Emperor, some other Jedi redshirts did. Even Windu (eventually). None of these characters really did anything unique....so, why are you motivated to like or even remember any of them again? Rather than single out guilty parties on this count, it's easier to just say Star Wars is easily the biggest offender, and other prequels (Ooh! I haven't mentioned "The Thing" yet.) really try to make the fact that everyone's on borrowed time into an interesting story, or hope that their prehistorical protagonist (Urgh, say THAT three times fast) isn't stamped "unkillable" too obviously.
Exception: I really didn't expect to be praising "The Temple of Doom" so much; Perhaps Mola Ram pulled some of that "Kali Ma" nonsense on me. Indiana Jones' earlier trek means that characters can be introduced on their own terms. Also, the one-off nature of this adventure, entirely unconnected to his academic and Ark-scouring episodes, means that no earlier characters have "friends" we'll never see again. Also, the main supporting cast (Willie Scott and Short Round) move on with their lives (we assume) just as the future Dr. Jones does. Here, though, they're perfectly fine (if a bit shrill or overly precocious - come on, you know which is which!) as characters in their own right for this chapter of Jones' life.