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.
Narm Charm: The entire sequence that follows Griffin first revealing he's invisible. From the police captain's underreaction, to Griffin chasing the cops around, to him stealing a bicycle and then throwing it back at the cops, it's all hysterically campy.
Much of Griffin's "Reign of Terror" is just childish pranks, like stealing money and throwing it to random people in the street.
Signature Scene: Griffin revealing his invisibility for the first time. You know, that scene where he unwraps his bandages while laughing maniacally in front of a horrified policeman and several terrified villagers.
Crosses the Line Twice: In "Possessed", Darien goes into Stage 5 Quicksilver Madness and decides to have a little invisible fun. One of his stops is a funeral, where he makes the body sit up and giggle around. Once everyone is scared away, Darien then steals the corpse's watch for good measure.
Harsher in Hindsight: In "Going Postal", Hobbes shoots up a post office, seemingly having snapped. It turns out to be a result of being exposed to drugs in a mail package. This was only a few months before the infamous 2001 Anthrax attacks through the mail. It was close enough to the mark that Sci-Fi pulled the episode from a scheduled rerun.
In Season 1's "Tiresias", Claire warns Darien that getting too much Counteragent will cause him to build up an immunity to it. That's exactly what starts happening in Season 2's "Possessed".
Magnificent Bastard: Arnauld manages to get Darien to rescue him in the penultimate episode and then manages to get access to Darien's laptop (with C4 stashed in it, which he uses to blast out of his cell) and then escapes Agency custody.
Also the Official is very good at manipulating everyone into doing what he wants.
Moral Event Horizon: Jarod Stark was always a villain, but him shooting Allianora in "It's a Small World" was this as far as Darien was concerned. After that, their relationship became far more personal.
"You did not need to DO THAT!"
Paranoia Fuel: What Allianora tells Darien in "Ghost of a Chance".
"Your country has enemies you don't even know exist."
Special Effect Failure: The villain in "Ralph" fires a pistol. There is no muzzle flash or recoil, and the action does not cycle.
Tearjerker: In the Pilot, Kevin dying in Darien's arms.
"The Other Invisible Man" reveals that one Simon Cole was the original pick for the I-MAN project—something he was reportedly very excited about. However, something went wrong, and he became permanently invisible—driving him insane and getting him shot when he attacked the Official. Darien eventually finds pages of Simon's journal, where the handwriting becomes more erratic because Simon could no longer see his own hand.
"I want my flesh back. I want everything back."
The Woobie: Darien. He gets caught in the pilot because he can't bring himself to leave an old man to die; he is kidnapped and forcibly operated on a few times by people who want invisibility.