One interesting 'Death Note Reaction Fanfiction' could be about the characters' reactions when they first discover the Death Note. For example, Light's initial shock and then his quick realization of the power it holds. Another might focus on how L reacts to the existence of the Death Note and Light's actions. There could also be fanfictions that explore the reactions of secondary characters like Misa when she gets involved with the Death Note's power.
When writing a 'Death Note Reaction Fanfiction', it's important to stay true to the tone of the original series. The Death Note world is dark and full of moral ambiguity. So, when writing about the characters' reactions, keep that in mind. For instance, if you're writing about L's reaction to a new lead in the Kira case, show his meticulous thinking process. You can also add your own twists. Maybe the Death Note has a side effect that makes the user see hallucinations. How would the characters react to that? Plan out your story structure, with a clear beginning, middle, and end, and make sure the reactions of the characters drive the plot forward.
Oh, for sure! Death Note is considered a masterpiece in the manga world. Its unique concept and the way it unfolds make it a must-read. The characters' development and the moral dilemmas presented make it stand out from the crowd.