Humor will only enhance a script. Often, a hot sounding idea just doesn't have the legs to be an entire movie. When this faulty avenue is pursued, all kinds of unpleasant things can happen. Stories begin to fizzle. I've read scripts where writers start new stories in the third act because they've run out of story in the second act.
Writers can be too truthful.
Yes, that's right. They fail to realize that movies are not real life. Real life is filled with boring passages, repetition and ambiguity. A good story should seem like real life, and it may embrace the footprints of real experience, but it's shaped by theatricality. Simple reality is not enough, you need a touch more.