King Gristle Sr. planned to feed Poppy, a baby at the time, to his son. When it's revealed the Trolls escaped, he tells the prince matter-of-factly that he will never be happy now.
Several of the monsters Poppy and Branch run into have frightening, off-the-wall designs. Of special note is the one that pretends to be a hill, which ends up eating Chef and Creek after the credits.
The reason Branch stopped singing. His singing as a child attracted Chef, and Branch's grandmother was eaten when she tried to save him.
After "Get Back Up Again", Poppy, who is unconscious and is wrapped in spider web, gets surrounded by spiders with all intent on eating her. Even after Branch shows up and fights them off, he still has to revive her.
The Reveal that Creek is a traitor is surprisingly unnerving, for all the wrong reasons. During the part where Creek says "I'm doing this for you", he strokes Poppy's face in a very rape-y manner, not helped by Russell Brand'ssoft-spoken delivery.