Aside from Gandalf's ridiculuous phyically assaulting Denethor,
the worst bit of movie Denethor was the lack of any
character development or even character collapse. Even in the
extended dvd bit when his boys take back Osgiliath Denethor
is portrayed as a thoroughly unpleasant person with no
discernable plusses.
There was no character arch,
which even movie Faramir had, to some extent.