This is one of the first big documentaries. Robert Flaherty travelled to Alaska to live with and capture the lifestyle of the inuits and this one family in particular. And you feel the human touch. The movie is so personal it's difficult to leave the film thinking "Wow that was an interesting documentary. I never knew this about that group of people." No, you are personally connected to this family on a level that you root for them despite the fact that it is a documentary. In a sense you could argue this was one of the first great documentary/narrative films that blended the genres where the line between it being a narrative and a documentary is blurred.
And what's interesting is that the manner in which this "documentary" uses title cards to advance the narrative and sequence the events together doesn't feel all that much different than narrative silent films which also had to rely on inter-titles to tell the story. In a way this film reveals how "documentary-like" most of the silent films were in nature, especially in requiring overt explanation of the story and context and why they are resounding to me, a modern viewer, as time capsules of pure humanity. Again, I will re-iterate that the scenes from some of these silent films, especially visible in Nanook, touch more of a realistic chord in my heart than most spoken modern films. There is some credence to the thought that silent black and white films were the truest in terms of realistic cinema, especially considering that the earliest films were just shots of people in their real settings being recorded with no direction, blocking, lighting, etc... and all these "narrative" elements developed out of that.
In this vein, Robert Flaherty continued to develop the medium of film out of the place of captured reality, blending authentic moments and narrative storytelling in a way that hits you on a very personal level.