I understand not everyone likes the same stuff, but I really love this manhwa for many reasons.
The cover arts are always gorgeous, as they always try to convey a set of dreamy images: a young girl growing up in a beautiful garden. The garden, presumably her guardian's, is also her secret garden of childhood. The unity in colors across the set of 6 volumes (green and yellow) are the symbol of nature, of green leaves, and of youth.
The art inside is also beautiful. The page layouts are well managed. They are of freer modern style, not the cramped old school page layouts like many other manhwa/manga/manhua have. I also love the artwork at the beginning of each chapter. They all show a fleeting moment of the characters: the guardian and the girl in the shadow of leaves, the girl and her new pair of high heel shoes, etc. The author is successful in portraying a doll-like child. The girl is truly adorable, both in her appearance, fragile - Yuri - glass, and her demeanors.
The name NOCTURNE implies the night time ambience. The manhwa really has some feeling of film noir without having the plot of a film noir. The series suits romantics like myself to a T. Who would not love that eerie moment of the uncle standing next to the window watching the young girl sleep at night? Or when the two of them driving in their car into the city lights? Yes, I'm totally in love with the imagery.
The plot itself may not be new, but it's not so predictable. The jumping back and forth in present time and the memories of the past also reminds me of modern writing. In fact, the story as is told from the perspective of a young girl in her own world is sort of a 'stream of consciousness' writing inspired by modernist writers. And yes, I understand the slow flow of the manhwa. It's about the progress of growing up. All the events add up to Yuri's own experiences, as well as her guardian's memories of the past. She the child growing up, and he the already grown up one.
I love the two main characters. The looks on their faces seem like they have a lot inside their minds without saying too much.
I love this manhwa for its feelings too. Being an introvert myself, I can totally identify with Yuri's character. The real romantic part is not about love, though I do ship Yuri and her guardian, but about her being in her own world.
People criticizes the guardian for isolating Yuri from the world, and his leniency kind of spoil her. Those are up for debate, really. I don't think Yuri will turn out well with that, but I don't think it's truly Dowook's fault either. He's a pretty chill grown up man.
And the romance part... That is too hard to say for now, because Yuri is still too young, but I love the chillily weird chemistry she shares with Dowook. But she also has that kid her age who is quite infatuated with her... Love triangle? I'm still waiting for the updates.