I had a simple question that I used to ask when I reviewed scripts on a now-defunct peer review site: "At what point does the audience become hooked into the story?" Now the conventional "wisdom" says it should be at the end of Act I (which is normally when the protagonist commits irreversibly to the quest). But I think that's too formulaic. If the audience gets hooked in as soon as the first page or two, what's the harm? BUT if the reader doesn't get head-over-heels drawn into the story by the midpoint (as was more often the case than you might have thought), then there was likely to be a problem with the entire story, no matter how enthralling the second half was.