Defending "Breaking Dawn"

It's started already, the bashing and trashing and thrashing of the next Twilight installment.  I just don't understand the hostility.  You hear very little fussing about the latest movie made from a Nicholas Sparks novel, which are incredibly formulaic and terribly sappy.  And tremendously popular.  But somehow acceptable in a way that Twilight just isn't.  What is it about this particular story that incites such fury?  It can't just be the fact that there's 4 books and 5 movies.  Look at any popular comic book franchise - Batman, The Hulk, Iron Man, now The Avengers, combining nearly a dozen superhereos.  There can be dozens of books, and many movies.  Look at the mother of all franchises - Spiderman.  They're "rebooting" the series yet again.  No one screams and cries and shakes their head and says "Why the hell do we need more of this?"  Every new comic book that's made into a movie is met with rapture, and no complaints.  Obviously every new Twilight movie is greeted with rapture, but also an outpouring of derision that I just don't understand.  It's not like they aren't making movies for other people.  It's not even like there aren't any other vampire movies, for those who are offended by the soft-pedaling of the genre - Underworld, for example just came out with the 4th in their series.  There's so many movies made for the fanboys, why are they so pissed off about Twilight?