The central idea: a villain has made himself invisible. You don't know he's coming until he's got his fingers around your throat. And he is furious. The scene where a man who was forced to help him runs panicked into a town screaming, "The Invisible Man is coming!" is still disturbing.
Griffin's other horrible trait is a lack of conscience: he has no problem killing or stealing from anyone, including his own father. He casually chats to a colleague about how he committed arson in a tone one uses to talk about the weather.
The care facility for insensates, people who lost most or all of their senses due to invisibility research. Particularly memorable is a man who keeps cutting himself with a shard of glass, implying that pain is the only thing he can feel anymore. When Darien takes the glass away, the man desperately gropes around for it.
In "Tiresias," Hobbes is attacked by a Quicksilver-mad Darien. Hobbes tries to call for help from a phone booth, as an invisible Darien smashes the glass and tips it over before trying to strangle him.
In the pilot, Darien wakes up in the middle of night, feeling what later turns out to be Quicksilver Madness. It's pretty unnerving to listen to his description of what he's feeling while looking at himself in the mirror in the dark.
"It was as if someone was knocking at the backdoor of my brain—something that wanted in real bad. Staring into that mirror, I had the sudden, horrible feeling that another person was looking back—a stranger that had stolen my face."
An invisible bigfoot in a permanent state of quicksilver madness. Enjoy your hike in the woods.