The Death Note anime generally covers the main plot points of the manga. However, there could be slight differences in how some subplots or character developments were presented.
The shinobi world's reaction would be complex. At first, there would be disbelief. Many would think it's some kind of new genjutsu or a hoax. But once they realize it's real, the more superstitious ninja would see it as a curse from the gods. The Hidden Sand Village might see it as a potential weapon against their enemies, while the Hidden Mist Village might be more cautious due to their history of bloodshed. The shinobi academies would start teaching about the Death Note as a new and dangerous threat. There would also be a black market for Death Note information, with some trying to sell false ways to protect against it.
Ryuk would probably just be amused. He is a Shinigami who has seen a lot in his existence. Fanfiction is just another strange human creation to him. He might chuckle at some of the more outlandish ideas in the fanfictions, like a character having superpowers beyond the Death Note.
Misa would likely be excited. She is a big fan of Light and the whole 'Kira' concept. In fanfiction, if there are scenarios where she has more power or a closer relationship with Light, she would be all for it. However, she might also get jealous if there are other female characters getting too close to Light in the fanfictions.
L is obviously one of the most important characters. He is a brilliant detective with an eccentric personality. His intelligence and unorthodox methods are key to the story. Near also plays an important role. He is another highly intelligent character who is involved in the events related to the Death Note.
The theme of sacrifice also stands out. L is willing to put himself in danger to protect the world from the chaos the Death Note can cause. There's also the exploration of power and how it corrupts. The power of the Death Note is so great that it can turn people into killers, and the novel shows the consequences of such power.