1. Faithfulness To Their Source Material
Probably the greatest sin of a bad Young Adult genre film is in its fidelity to its source material. Young Adult films only really get greenlit because of their amount of fans, and the same fans often prevent the directors and screenwriters behind the adaptation from taking any dramatic changes that may even improve the source material - or at least make it work better on screen for what it is. No matter how "good" anyone thinks the book is, there is no work of literature on planet earth that is so perfect it can be adapted straight to film without any changes. Rumor has it there exists an early draft of the Twilight screenplay featuring Bella as a neo-Buffy action heroine, fighting back against the immortal forces seeking to control her life. No doubt it was revised out of fear of a copyright lawsuit from Joss Whedon, but lawsuit or no, this would have made a far more interesting film than the one eventually produced - rewritten with painful fidelity to its source material. Despite the derision and outright anger many movie fans have toward bland Young Adult films, that is all they are - bland. The failure of these films is not being awful, for that at least would have been interesting to watch for anyone who doesn't own an autographed copy of the novel under their pillow. The failure is in how uninspired they often end up being. Without creative vision or risk-taking adaptation to develop themes or characters, the end result only makes any money at all because of a fan base, not any cinematic merits at all.