To quote Whedon:
"It wasn't a question of doing everything differently, although they changed the ending; it was mostly a matter of doing everything wrong. They said the lines...mostly...but they said them all wrong. And they cast it wrong. And they designed it wrong. And they scored it wrong. They did everything wrong that they could possibly do. There's actually a fascinating lesson in filmmaking, because everything that they did reflects back to the script or looks like something from the script, and people assume that, if I hated it, then they’d changed the script...but it wasn’t so much that they’d changed the script; it’s that they just executed it in such a ghastly fashion as to render it almost unwatchable."