First, look for how well the characters of the golden trio are portrayed. They should stay true to their original personalities. For example, Hermione's intelligence and determination should be intact. Second, the 'lemon' part should be integrated in a way that doesn't seem forced. It should enhance the story rather than just be there for shock value. And third, a good plot that takes advantage of the trio's relationship and the magical world they inhabit.
One important feature is the balance. There needs to be a balance between the regular adventure and the 'lemon' aspects. You don't want it to be all about the latter. Another is the character growth. The golden trio should grow and develop in the fanfiction. For instance, Ron might overcome some of his insecurities in a new way. And finally, the world - building. The fanfiction should expand on the magical world we know in a believable and exciting way while still being centered around the golden trio.
The key features include a proper exploration of the dynamics between Harry, Ron, and Hermione. In a great 'golden trio lemon fanfiction', their friendship should be at the core, with the 'lemon' elements adding an extra layer. Also, the writing style matters. It should be engaging and descriptive enough to draw you into their world. The fanfiction should also respect the lore of the original series, so any new elements or twists should fit within the framework of the magical universe.