The Grey/Nightmare Fuel

The Alaskan environment itself trying to kill them.
That loud roar the Alpha lets out when Diaz was taunting him after killing one of the wolves.
The moment near the beginning when they realize how severe the wolf problem is. It's night, they hear something move in the distance, walk over with torches in hand and look into the darkness. All of sudden, about SEVEN pairs of eyes come into view, not attacking them but just watching and waiting. Similarly, after Diaz taunts them as mentioned above, you can see the breath of the Alpha in the moonlight. Then it's joined by the breaths of all of his packmates.
The scene where Talget was preparing to cross the ravine. An extended shot of him trying to unfog his glasses and psyche himself up, with no music playing. The scene is identical to previous scenes where a wolf suddenly jumped out and mauled the person to death. The tension could be cut with a knife.