Well composed, likeable and intelligent characters, and a truly scary threat.
The amount of detail in the writing really helps to connect you to the locations and equipment that the characters come across.
It is also extremely refreshing to read a story with characters whose actions are largely well thought out and logical and spend their few safe moments talking and planing instead of fighting and posturing. I genuinely cared and feared for the characters.
The narration was great. I felt like the narrator himself was one of the characters, recounting the harrowing events some years later.