Origin stories are boring.
Okay, that's not entirely true. The Secret Origin story is a staple of comics, and as a kid I remember learning the origin stories of super-heroes that I liked and being totally stoked. The difference is that these were origins for characters I already knew. But starting off with a long, involved origin for a previously unknown character is a sure way to boring the readers.
A perfect example of this is in the first Araña collection from Marvel, which I just read a couple of days ago. It tells the origin of the titular hero, and takes an entire six issues in which to do it. She doesn't even appear in her super-heroine outfit until the final page of the collection.
This is the kiss of death for a monthly serial. The origin is not the interesting story; it's background information. If the information in the origin is important to the story you're telling, then you can go back later and fill in for the reader. But don't start with an issues-long origin.
The original appearance of Superman disposed of the origin story in a couple of panels. It was only later, after Superman became an icon, that his origin was fleshed out (and out and out, ad absurdum).
Or take for example the modern-day classic, Watchmen. Alan Moore created a rich background mythology for the characters, but he starts out the story with the investigation of a murder; in other words, he starts the story at the begining of the actual story; all the background was filled in later, as needed.
But you don't have to be Alan Moore to do this right. Brian K. Vaughan understood this with his recent series Ex Machina (now in an inexpensive trade collection--go buy...) It's the story of a former super-hero who becomes mayor of New York, so the first issue starts with Mayor Hundred in his job. We are slowly being given the origin (both how he became a super-hero and how he became mayor) through flashbacks. Same with Vaughan's Runaways--by the end of the first issue, the kids have learned that their parents are evil super-villains and are on the run.
Super-hero movies are just as guilty as comics. I love the first Superman movie to death, but it takes well over an hour of Krypton and Smallville and the Fortress of Solitude before Chris Reeve dons the familiar red-and-blue tights and takes to flight. The first Spider-Man movie was good, but the second was even better, because the producers didn't have the need to show us the origin again. It took too long for the Green Goliath to appear in the Hulk movie, when let's face it, most people were there to see a big green CGI guy throw tanks around.
The super-hero movie that got it just right was Tim Burton's first Batman. We quickly learn that there's a guy who dresses up like a Bat and roams the city at night, beating up on criminals, and that in the daytime he's Billionare Bruce Wayne. Sure, later on we learn that he became Batman to avenge the deaths of his parents, but even then the movie doesn't dwell on it.
In contrast, the upcoming Batman Begins looks as though it's going to dwell on the Batman origin, which doesn't look too promising. Same with the Fantastic Four movie.
Think of all the good genre movies you've ever seen. How many begin with a long origin sequence? Did Raiders of the Lost Ark start with 45 minutes of young Indiana Jones getting his PhD in archaeology? Did Star Wars begin with the origin of Darth Vader?
Origin stories are not the point; they're background and should be treated as such. It should not take six issues to give us the origin of a character. An interested reader wants to read stories about a teenage girl with spider-powers, not a long drawn-out story about how said girl gets her spider-powers.
Because origin stories are boring.