This anime is a panacea of the heart and mind.
Life too stressful? Worried or anxious about something? Just sit down in front of the screen and put this anime on, and all your concerns will dissolve away - probably within seconds. Everything - setting, plot, sound, music, art, humour - all of it is perfectly crafted at maximising this effect. And what an effect it is.
The story itself is a simple one. It follows the daily lives of four girls of various ages who all live in the same rural village and are in the same class at school. Almost everything that happens is following their daily lives, with events that are both realistic and completely believable. As far as overarching story goes, that's about it - this isn't an anime you should be watching if you only care for a tense plot full of twists and turns. It is predominantly episodic in nature, although characters and settings are gradually built up over time so there's no lack of progress. Each episode contains one or two stories, every single one of which is a joy to watch and will melt your heart. The humour is lighthearted in tone, and is aimed at adding to this heart-melting effect, rather than at making you laugh out loud, although there are a few moments that could do this as well. There are also a few moments that will invoke other emotions, all very effectively and very naturally.
The cast is focused mainly on the four girls. Komari is the eldest of the four, although you wouldn't know it by looking. She is very self conscious of the fact that she is unusually small for her age and tries to make up for this by attempting (and largely failing) to act grown up. Hotaru, on the other hand, is unusually mature, at least on a physical level, for a fifth grader. She also quickly develops a crush on her cute chibi senpai, although none of the others ever seem to realise it. Natsumi is Komari's younger sister, and is a bit of a layabout. She's too lazy to work hard at anything and is always getting in trouble for one thing or another. And then there's Renge, called "Ren-chon" by Natsumi. I could honestly wax lyrical about Ren-chon all day, but I don't want to bore you any more than I already am, so I'll just say that she's the youngest of the cast, who knows a lot about things that you wouldn't expect a first grade student to know, but otherwise acts fairly normally for her age. Oh, she also has some brilliant facial expressions, and Koiwai Kotori's voice acting of her character is a definite candidate for being the best I've ever come across.
Every member of the supporting cast, just like the main characters, each have their own distinctive but realistic and believable personalities, and are every bit as likeable as the central four cast members.
The humour and characters may be brilliant, but what makes this series really stand out is the music and images. As someone who tends to focus very heavily on the characters of a series, I don't usually care much for these things - as long as they add to the immersement, enjoyment and feel of a series, just about anything's fine by me. But here even I can't help but be overwhelmed. The series starts with nothing but pictures, sound effects and music building up the scene and setting with pure imagery. It's almost 2.5 minutes before anything is actually said, yet by that time you will likely have already entered into a state of total bliss from the gorgeous artwork, more reminiscent of a high budget film or even something you might see in an art gallery than a TV series, and soothing background music and sound effects of what is being shown. And, while the effect may not be as intense in later scenes, the quality does not drop. Just about every background in this entire series is a work of art both literally and figuratively.
I've watched the first episode over and over again and every time the effect is the same. The entire anime is thoroughly rewatchable, and every episode provides the perfect relaxation whether on your first watch or your tenth. Even brilliantly written series that depend on plot twists and reveals often grow dull when watching through for the third time, but that is of no concern for an anime like this.
A lot of slower paced anime, however, require you to be in the right mood before watching them, but again that is of no concern here, as Non Non Biyori will put you in that right mood, no matter what your previous state of mind.
All in all, this is a series to be watched, loved and enjoyed over and over again. Everything is perfectly toned to add to this viewing pleasure as much as possible, and it is indeed a pure pleasure to watch.
If you want to know what nirvana feels like, watch this anime.