A good Denmark x Norway yaoi fanfic often has well - developed characters. It should capture the essence of both Denmark and Norway's personalities as they are in the original source. For example, Denmark's boldness and Norway's more reserved nature.
The writing style also matters. A great fanfic would have a smooth flow of language, engaging descriptions, and realistic dialogue. It should make the reader feel like they are really there with Denmark and Norway, experiencing their relationship unfold. Also, a good fanfic might include some cultural references related to both countries, which can add more depth to the story.
The plot is important too. It could be a story that explores their relationship in a unique way, like how they deal with historical events together or how they interact in modern - day settings. Maybe a story about them going on a trip together and how their relationship deepens during that time.