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.
Ensemble Darkhorse: Sark was supposed to be a one-off character, a gofer for K-Directorate or Irina or the Alliance. Fan reaction to David Anders was so great that he became more and more high-profile, eventually taking over the Big Bad mantle in the series finale flash-forward.
Evil Is Cool: Sark is the epitome of this trope. He's young and attractive, he frequently wears expensive, elegant suits, and his dry sense of humour produces numerous amusing quips. Julian even sports stylish sunglasses to enhance his villainous awesomeness.
Fan Nickname: Fans nicknamed the nameless recurring scientist and torturer "Suit and Glasses" or Television Without Pity's name for him "The Sadistic Dentist of Asian Persuasion".
"Suit and Glasses" was the character's official name until his real name was given a few seasons later.
Jack Bristow is also often referred to as "Spy Daddy" by fans.
Ditto Irina Derevko as 'Spy Mommy', Syd as 'Spy Baby'/'Spy Barbie', and Nadia as "Spy Skipper".
Once Francie was killed, and her identity assumed by an identical imposter, she went by the name Evil!Francie or The Francinator.
Foe Yay: A rare, married couple example - Jack and Irina. Syd/Sark and Syd/Anna also fall under this category.
Ho Yay: Between Vaughn and Sark mostly. Though Sark also managed to get his yay on with Marshall at one point.
Weiss and Vaughn were also a very popular pairing. Check out the below exchange for proof:
Weiss: Okay, whenever you want to have that talk...
Vaughn: What talk?
Weiss: About the cologne or whatever the hell you're wearing.
Vaughn: I'm not wearing cologne.
Weiss: Well, something's going on. No man naturally smells as good as you do right now.
And Jack and Weiss. And Jack and Will. And Jack and Vaughn. And Vaughn and Will. And Will and Sark. It really got ridiculous.
Dr. Brezzel's assistant, Kaya, looked at Sydney (and everybody else for that matter) like she wanted to tear her clothes off.
Jumping the Shark: Most everyone agrees that the show did this at some point. No one agrees exactly when, with opinions ranging anywhere from the middle of season five all the way back to the big Time Skip in the last scene of season two.
Moral Event Horizon: Sloan crosses it in the first episode by ordering the murder of Sydney's fiance, but keeps going. Killing Dixon's family in retaliation for Dixon accidentally killing Emily is another. Accidentally throwing Nadia through a glass table and killing her is a third.
Narm: Vaughn trapped in the compound in Taipei, during "Almost Thirty Years", complete with slo-mo drowning, and yelling at Sydney to leave.
Sydney going the full Wig, Dress, Accent route in the season 3 premiere for absolutely no reason. She develops a full alternate persona just to walk up and shoot a guy.
One-Scene Wonder: The show had many, many memorable guest stars. Quentin Tarantino, visibly enjoying himself as McKenas Cole. In Season 1, he's more of a Two-Episode Wonder, but when he pops up again in Season 3, he fits the trope perfectly.