These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Bigger Bad: There's no way that the parasites can be natural creatures. The fact that they are driven to devour whatever species their host is makes no sense evolutionarily; such a species would soon run out of hosts and go extinct. This, coupled with the sheer specificity of the parasite's goal, which is based off an ideal, not a natural selective pressure, means that someone had to have created the parasites, with the goal of annihilating humanity in mind. I always assumed that the parasites were aliens, or at least created by aliens, but this makes little sense. After all, why would an alien be concerned with a planet which is not their own? Perhaps the parasites were invented by a human being, some scientist with an apocalyptic view of the world. Certainly the voices at the beginning of the manga (which must belong to the parasite's creator) are voicing common human ideals about humanity's impact on the world. There's also the possibility that the parasites were invented by god, sort of a plague punishing humanity for destroying the world.
Complete Monster: Uragami is a human being who manages to be many times worse than the man-eating parasites of the story. A sadistic Serial Killer, he considers other people his "playthings" and gets off on all acts of cruelty he can inflict them, including murder, rape, mutilation and cannibalism. After he was caught and forced to work for the army, he gleefully helped them slaughter dozens of parasites along with a few innocents before killing the police officer escorting him and escaping. Knowing that it was only a matter of time before being caught again, he decides to spend his last days as a free man forcing protagonist Shinichi, who had never done anything to him, to confront him. He does this by killing more innocents, capturing Shinichi's girlfriend Murano and threatening to "have a lot of fun breaking this little doll" if he doesn't comply, only to throw her from a rooftop and stabs Shinichi when he does. A depraved fiend, Uragami was even able to see through the parasites' disguise because he was, like them, a monster hiding behind a human appearance.
HSQ: Parasyte could easily be renamed to "HSQ: The Manga".
Narm: When Shinichi is at death's door, Migi fills the roles of trauma surgeon, nurse, AND organ donor in order to save his life. It should have been the dramatic turning point of the story in more ways than one, but thanks to the visuals that accompany this scene... not so much.
Uncanny Valley: In the story, Parasites generally are indistinguishable from normal humans. However, they tend to have completely blank expressions due to not having emotions, and the artwork often gives them slightly overlarge and sharply-defined eyes that give them a sinister appearance. Extremely twisted humans, such as Uragami and Takeshi Hirokawa, also have these eyes. There's also Sanji.