Dangerous Sexuality
The purest character in the film, Ellen, is portrayed as someone free of sin and, potentially, a virgin. The love between her and her husband Hutter is more like pre-teen puppy love than an adult, married relationship. Contrast her innocence with Count Orlok's dark eroticism and clear bisexuality. The way he slurps blood from the cut on Hutter's hand seems to disturb Hutter mostly on account of the action's sensuality. At the end of the film, Orlock approaches Ellen in her bed, and while we know that he wants her blood, Murnau's direction suggests he seeks something else too. Notably, Murnau was a closeted gay man for most of his life, and his struggles coming to terms with his sexuality in a prudish Germany can be felt reverberating in the vampire's dual portrayal as seductive and vile.
Xenophobia
Count Orlock and his rats represent a foreign threat. It's clear that the town of Wisborg falls because of the descendence of some other from the east. Orlok is portrayed as something of a caricature of an Eastern European (or even a Jew), with his hook nose and hair sprouting from odd places. Much of the horror of the film comes in the form of the rat-carried bubonic plague, drawing a parallel between the illness carried around medieval Europe by infected rats on ships, and the influx of Eastern Europeans in Western Europe. There's no reason to believe, though, that Murnau himself was xenophobic or that Nosferatu is a xenophobic film. Remember the context. This film was made just a few years after the end of World War I, which was instigated by a Balkan group assassinating the Austro-Hungarian emperor—an Eastern European upending a Western seat of power.
Morning and Night
Murnau's take on German expressionism is not one based on set design and strange geometries, but one that portrays the world around the film as reflective of the film's genre. This take on expressionism plays out on a narrative level, with a constant tension between the late night and the early morning. The morning scenes are often ones focused on Ellen, from the opening, where she plays with the kitten in the garden, to the very end, when she keeps the vampire awake until daybreak. late-night scenes are the domain of the vampire, as that is when he preys on his victims and undertakes his travels. Hutter lives out this tension between night and morning, exhausted as the film goes from his own sleeplessness.
The Plague
In Nosferatu, the plague is an evil that can destroy an entire population. It is portrayed as a reprisal of the bubonic plague that killed millions in the medieval times, but it also stands as a meditation on a different force that wreaked havoc in Europe much more recently. Murnau uses this epidemic as an opportunity to meditate on the swift and totalizing destruction caused by World War I, and likely much of the terror that he derives from that epidemic was inspired by the fresh memories of a war in which he himself participated. Germany was forced to shoulder responsibility for the war, so the audience viewing the film was one still quite haunted by WWI.
The Occult
Prana Films was a short-lived production company, declaring bankruptcy right after Nosferatu was released, in order to avoid paying out a settlement to Bram Stoker's wife for copyright infringement. But the enterprise was developed with the intention of making occult films, and that theme is explored exquisitely in this one. Murnau developed that otherworldliness through special effects, such as the vampire levitating and vanishing. He also revels in the idea that an occult world is one just out of sight: creeping below the deck of a ship, inhabiting some castle seen from the distance, lurking in the abandoned building across the river.
Money Is the Root of All Evil
The original sin in Nosferatu is greed. Knock doesn't have to try very hard to get Hutter to travel to Transylvania and sell Count Orlok the abandoned building across from the Hutter residence. All Knock has to do is promise him a serious payday. Knock is really only portrayed as somewhat sane while playing the role of realtor, but that veneer crumbles the second he is removed from his office. We come to see this business person as a raving lunatic, eating flies and ready to kill. Of course his master, the vampire Count Orlok, is a wealthy man. There's no question that money and evil go hand in hand in Nosferatu.
The Non-Human
Some of the most affecting shots in the film are Murnau's nature photography: shots the looming mountains and sprawling fields, of the horses running in fields and Hutter struggling to pass through a rocky river, or the choppy seas the ship traverses. Through these images of the non-human world, Murnau makes our characters and their lives seem tiny, even vulnerable. There are forces out there that dwarf them and make them powerless. This sets the stage for the otherwordly terror to descend on Wisborg, since we know that the city, and all of the humans in it, are just a part of some grander equation.
