I think my problem is that they're picking the worst book to stretch out into two movies. WHen I read the book the first thing I thought of was all the padding was put there to cut it out for the movie... I can't imagine a movie of that padding.
It may be that they need the extra time in order to add in further details that previous movies dropped, I think Order of the Phoenix or Half-Blood would make a two movie split better. I think either way they'd be better off making the movie as one normal length movie, then having a Super Extended Edition DVD.
The other thing is, they always talk about having to trim materials, but most of the time my problems have more to do with what they add. If Goblet of Fire needed more time, fine, but then extending the dragon chase into an idiotic aerial affair seems out of place.