I think I had this on my Top ten for 2006. It's definitely one of my favorite films.

What i liked most about it was the almost casual pace of the film, the many seemingly unimportant scenes and moments that lingered on - both of which would have usually been edited out of less observant films. This is definitely something Kim has in common with Jia Zhang-ke. But i think her visual style is also more direct and straightforward, at first glance less lyrical and epic than Jia's. The unobtrusive and gentle look at the protagonist is also very unique I think. If she speaks about Dardenne and Tsai there are obvious connections, but I must say that she definitely has her own way of showing things, which I found as good as the one by the three filmmakers mentioned (and in some moments even better). I think her view is in a unique way gentle and bleak , and the places and situations presented were mostly pleasing to me. Maybe this has to do with the fact that I love snowy landscapes in big cities, and I could identify with the feeling of having moved to one. I think the film is difficult to grasp, and one of its qualities is also its elusiveness. 
The unsettling and abrupt ending of the film is also in a league of its one, and was my favorite moment when I saw it for the first time. Very fitting, and of a complexity one usually doesn't encounter on the screen (again, Rosetta does come to mind). 

Somehow I enjoyed practically every artisitc decision Kim made in her film. The telephone conversations which could easily sound uninspired were surprisingly also a joy to watch.

I can't exactly say why the film is so good, but you are quite right in your observations arsaib. It's not so much the coming-of-age than the whole composition of the film. The town, the season, the love-interest, the friends, the loneliness, the absent father, the relationship with the mother, and not least the protagonists eyes, face and movements. Her expressions are in my opinion incredibly effective. The cinematic sculpting of time and space coupled with a subtle narrative make the film more of a documentary-inspired portrait of a short time-span in one person's life. The coming-of-age is a consequence rather than the focus of In Between Days. Like a diary that is regularly missing a few pages, the film does show what usually happens in between the so-called "events" in one's life. 

There are parallels to the films of Angela Schanelec, especially the elliptical nature of Marseille (2004), a film that almost never shows the events themselves. What we see is always what happened before or after. What has or hasn't changed, and the difficulty of grasping the importance of moments that define our lives.