- Smoking—A miscellaneous character smokes something in a bar/club that could be construed to be drug-related, but it's never identified one way or the other.
- Blood/Gore—River has a small red hole or mark on her forehead from where a device was attached to her.
- Frightening scene—Simon applies a shot to Mal's forearm, but we don't see the actual insertion.
- Frightening scene—The crew endures a rough and shaky landing in their spaceship.
- Imitative behavior—[Wash says] "I'm a leaf in the wind."
- Music—A heavy amount of suspenseful, dramatic and action-oriented music plays in the film.
- Sex—We see a miscellaneous shirtless guy.
- Violence—Mal and Jayne throw grenades at approaching Reavers, but we don't see the explosive impact.
The author is totally right—the PG-13 Serenity definitely includes a lot of mature content that a thirteen-year-old child wouldn't be able to handle.
(Those, of course, were carefully selected for my own amusement. I left out the parts about vibrator use, the prostitute training school, machine guns used on zombies, and the constant flashes of pictures of rotting corpses: things that parents really might not want their young kids to see.)