Plot contrivance: When you adopt a baby from another race, it loses all its racial characteristics and features...but slightly gets them back when it is angry. So either Loki never got mad before the events of the movie, or no one noticed his skin change color when he did.
Plot contrivance: Loki freezes Heimdall but does not kill him. Afterwards, Loki tries to kill Thor but does not freeze him. An interesting fact to know is that if Loki killed Heimdall or froze Thor, he would have won.
Plot contrivance: Odin protects the Frost Giants' casket by putting it on display in a treasure room with the Destroyer hiding behind a wall instead of being in front of it. Even better yet, why not put the casket inside the Destroyer?
Unaddressed Issue: 1000 years ago, the Gods of Asgard fought against the Frost giants of Jotunheim and seized the source of their power: the Casket of Ancient Winters. In present time, the Frost Giants still look as menacing and strong as before even with the lack of access to their source of power for more than a milleninum. Why?