I've only read the first two volumes so far, but here's the best answer I've yet seen to the question Why is Knausgaard so great - when really, with all the detail and mundane plotlines, he should be boring:
The answer lies not in Knausgaard’s depth of revelation so much as the intensity of focus he brings to the subject of his life. He seems to punch a hole in the wall between the writer and reader, breaking through to a form of micro-realism and emotional authenticity that makes other novels seem contrived, “made up”, irrelevant. As [Zadie] Smith put it: “You live his life with him. You don’t simply ‘identify’ with the character, effectively you ‘become’ them.”
There's so much talk about literature's ability to put the reader in someone else's head - this is often portrayed as the feature that most differentiates writing from other forms of art, and Steven Pinker even singled out the increase in putting-yourself-in-other-people's-heads caused by the invention of the printing press as the trigger that started the long-term decline in violence ca. 1500-2000. I've read a fair bit of fiction and some memoirs, but have never seen anyone doing it like Knausgaard does.