The story follows a bunch of people in their early 20's, showing both their struggles and happy times in a believable, relatable way. The show manages to be both funny and sad, just like real life, and it managed to make me cry more than a handful of times (which is saying something, because I don't cry easily). You would think that it would slide into a soap opera after some time, but nothing is ever over the top. And it doesn't shy away from adult topics either.
While there are a few derpy moments now and then, overall the animation and character design is highly realistic. The characters wear different clothes every day, their make-up doesn't make them look like clowns and they look different when they aren't wearing any, and their faces are really expressive. There is some nudity in the show, but it is presented very naturally.
The voice acting is absolutely amazing. It especially shines in the narrative moments, where Kaori and Romi Park use softer voices. And the soundtrack is brilliant as well, including all of the opening and ending themes. It really helps set the mood, be it in humorous or emotional moments.
The characters are layered, complex, and realistic. Every single one of them has strengths and flaws, is completely unique and human, and makes mistakes. I honestly couldn't bring myself to hate anyone, because I could understand where they were coming from, no matter how "bad" their decisions may have been. Takumi, for example, I find to be one of the most intestesting and well written characters, although if he was in any other show I would most likely have hated him. But here, I simply can't.
I've seen a lot of anime over the years, but I still regard NANA as the best anime I've seen. It's a unique experience, unlike any other, and everyone should at least give it a try.