Skulker at first was the most dangerous enemy Danny encountered, but in his following appearances, he became less and less a serious threat, only showing competence one or two times during his encounters with Danny.
Vlad also suffered from this. In the beginning, he was shown to be the most powerful enemy Danny had, being able to easily defeat Danny with one hand behind his back. However, such success diminished overtime when he encountered the likes of Pariah Dark and Vortex. Even a few times, Danny was able to beat him.
Sam Manson. She is a polarizing figure due to being bossy and hypocritical at the eyes of many fans, usually as a means to push her agenda; temporarily forcing her Ultra-Recyclo Vegetarian diet on the entirety of Casper High or protesting a beauty contest, and criticizing Danny's power abuse, but supportive when she wanted to scare people away from buying environmentally threatening trucks. However, her fans worship her and romanticize her (eventually canonical) relationship with Danny. The YouTube video "The Rise and Fall of Danny Phantom" demonstrates this perfectly. While author Alpha Jay clearly doesn't like Sam, guest Kuro the Artist is an obvious supporter of hers.
To some extent, Danielle "Dani" Phantom. Part of the fans think she is a CanonCopy Cat Sue with no interesting personality, other thinks she is an interesting character with underexploited potential.
Is Tucker funny or annoying?
Big Lipped Alligator Moment: The random iguana-ghost attack during the beginning of "King Tuck". It's never mentioned again for the rest of the episode.
That might have been used to represent how no one listens to Tucker.
Canon Fodder: Lots, the most famous being Dark Danny's ultimate fate.
Cliché Storm: This series takes every "superhero in high school" trope and somehow makes them work. Helps that the writers lean into the absurdity inherent to the series and regard the whole thing as an Affectionate Parody of the genre.
Vlad is the second most paired off character in the series. The fact that he has plotted and would commit murder, was responsible for many people getting hurt in attacks and Dark Danny in one timeline doesn't matter much to most of his fangirls. They see (as the entry puts it) a "...suave, handsome, charismatic, obscenely rich, relatively competent, with an alternate form heavily resembling a vampire and superficially sympathetic motives (he wants to make his old love interest his wife and his arch-nemesis his son): a perfect object for fangirl lust..." Though one could argue that he's a borderline deconstruction. When Danny goes back in time to reverse the accident that caused him to become a ghost, he's villainous regardless.
Dark Danny also has his fans. Most notable thing about him is his voice and body, but most fans tend to forget he's a mass murdering psychopath that would as just as soon do this to them as kiss them... if they're lucky. Remember he is a sadist.
According to fans, the Ghost Writer is one Hot Librarian. Somewhat averted in that he wasn't evil, just overzealous in trying to teach Danny a lesson.
For the boys, there's Desiree and Ember.
Ear Worm: "He's gonna catch 'em all 'cause he's Danny Phantom."
Ensemble Darkhorse: Clockwork. For a character only appearing in two episodes, he has amassed quite a following.
Same for Ghost Writer, especially with the fangirls. One episode and there are SWARMS of fanart of him everywhere. Don't even get me started on Dark Danny.
The fanboy equivalent would be Ember. Gothy, leatherclad and clearly desperately lonely--not to mention having a very ambiguous backstory.
And Desiree as well. Although a bitch, it's sort of justified in that her sultan's lover was an ass (so of course we feel sad for her)--plus, have you seen that body?...even though it's in 2D?
Esoteric Happy Ending: A large complaint about "Phantom Planet." Despite being said to wrap up every plot point, it created several more and left several lingering plot points (such as Danielle and Valerie) hanging.
Evil Is Sexy: Vlad Masters, adorned in a black suit. Also, Sam during her temporary evil in "Urban Jungle".
Also Dark Danny, whose voice acting was so brilliant it stands out in fandom. It even gained him fangirls. Also, that muscular body is a very large improvement for someone who did it in just ten years.
Fan Dumb: Oh, so much: The ongoing True fan/Anti-fan wars, The people who act as though Danny's a real person and get annoyed with the way he's treated in fanfiction -particularly Vlad/Danny slash, which admittedly is pedophilia, the general air of Serious Business....need I go on?
Fandom Specific Plot: Either this or Character Shilling. Danny dates a girl from another property, usually having many other girls fall for him in the process, faces off against a returned Dark!Danny and proves to be the most powerful being on Earth, being heralded as the greatest man who ever lived. Dash, Paulina and the other A-listers are humiliated while Sam and Tucker are mocked as utter losers whom no one should touch with a twenty foot pole.
Most Danny/Sam shippers believe Danny's affectionate nickname for Sam is "Sammykins". The only time it was ever used in canon was by her mother when waking Sam up.
Given the Flip-Flop of God relating to what the ghosts are (Seasons 1 and 2 painted them as deceased spirits, Season 3 had them be simple monsters from another dimension), most fans say that both types exist. Someone like Ember was the first while Skulker is the second.
Jack: An old friend? No. You? Yes. *Turns his back and leaves*
Danny getting his powers back in "Phantom Planet".
Danny: *To other ghosts* The good news is, my powers are back. The bad news is, my powers are back!(KO's everybody with the Ghostly Wail)
Jack gets another one in "The Million Dollar Ghost" where he single-handedly thrashesVlad.
Vlad: This can't be! You're an idiot! An idiot!
Jack: That may be, but I'm the idiot who beat you.
Motive Decay: Vlad's initial, intriguing motive to turn Danny into his son and apprentice, putting him on par with Slade from Teen Titans, disintegrated without explanation by Season 3, leaving him nothing your average, run-of-the-mill Villain with Good Publicity.
OC Stand In: Given how underused Danielle was, it's not uncommon to find her in this role.
Periphery Demographic: "Boy's Action Show"--ya right, Hartman. The fangirls of mostly the teenage and college age variety eat this show up.
Given her Base Breaking status, it's very easy to find fans who play up Sam's flaws.
Tucker gets this as well but he's usually treated better than Sam.
Dash and Paulina are hardly saints but fanfics, crossovers in particular, really amp up how vile they are.
Seasonal Rot: Despite the introductions of new friends and foes, the development of new powers, and the return of some fan favorite characters, many felt the third season went through a bit of this due to changing of executives and the very bad writing plots of many episodes ("Urban Jungle" is one of the most prominent examples).
Scapegoat Creator: A surprisingly large amount of the fanbase has nothing but disdain for Butch Hartman.
Fan-Preferred Couple: Danny/Valerie, Danny/Vlad, Danny/Dan, Sam/Tucker, Tucker/Valerie, Sam/Dan, Sam/Kwan, Sam/Dash, Danny/Tucker/Sam, Maddie/Vlad, Danny/Paulina - the list goes on and on and on and on...
That's not to say Danny/Sam isn't popular. In fact, at least a quarter of the fandom ships them.
Ship-to-Ship Combat: The infamous "True-Fan VS Anti-Fan" wars which basically boiled down to "Canon" (Danny/Sam) VS "Fanon" (Danny/Anyone Else) and/or "Heterosexual Pairings" (IE: Danny/Sam, Danny/Valerie) VS "Slash" (Danny/Vlad, Danny/Dash, and even Danny/Himself *Don't ask*).
What is even more disturbing, one has multiple options for the last one.
Tough Act to Follow: Any work Butch Hartman makes will inevitably be compared to this. And not well.
Values Dissonance: In "One of a Kind", Sam blackmails Danny and Tucker into helping her by threatening to release a picture of the two accidentally cuddling. By The New Tens, it's doubtful that most of the school body would give a damn if the two were gay.
Villain Decay: Vlad during the first two seasons was a Magnificent Bastard and a very competent arch-foe with an often sympathetic side. But in the third season, he became a shallow crook with little redeeming qualities; his final plan was to force the world to let him save it from a giant meteor in exchange for world domination (an agenda that was poorly conceived) and 500 billion dollars...as if he wasn't already filthy stinking RICH!
Though to some degree it was justified. He underwent a Villainous Breakdown after his perfect clone was destroyed and he didn't want Danny as his son any more. So thus he was more interested in making Danny's life a living heck than his original motive.
The Fright Knight suffered from a mild case of this. In his first episode, he's the most powerful and terrifying villain Danny had ever faced; in later episodes, he's always relegated to the role of The Dragon. He's still just as powerful as he always was, but seems weaker in comparison to Pariah Dark (and later Dark Danny).
Vocal Minority: There are those fans who are a bit too eager about the show and how great it is which mainly serves to turn off potential fans or even other fans.