I think we can ascribe this to a couple of factors:

1) Disconnect between the audience and the writer as to where things were going. Namely you as the audience expect certain things and begin to write the story yourself in advance then feel inherently disappointed when you brilliant idea is not taken up.

2) The writer changed their mind in the process of writing. This is both positive and negative. Sometimes an idea in the writer's head (or bullet point idea) doesn't work out on paper, or they think of something "better" later on. Or alternately writer's block, they legitly didn't know where they were going and ended struggling to put anything out.

3) External factors. An outside force interferes with the story, maybe executives demand action and/or romance in the story, or the director of the film doesn't agree with the script, or the script is a collective work that passed through multiple parties in the first place

Quote Originally Posted by Serpentine View Post
Well, it's been a while so I can't remember exactly, but the biggest one is probably that he's still a pig. Maybe that's not a bad thing, but it's a long way from the sort of story I'm used to.
Interestingly Miyazaki wants to do a sequel to that one. I'd also point out that Porco's curse is not really the point of the movie and more of an underlying theme.