Alas, Poor Scrappy: Dexter wishing away Koosalagoopagoop, and he reluctantly complies. Even if you didn't like the character, you can still feel pity for him, and tell why Dee Dee and even Dexter were upset at it happening.
Quite a few fans believe he has Asperger's syndrome, or something similar.
One could argue that Dexter's Mom has a form of OCD, especially if the Mother's Day episode is anything to go by.
Dee-Dee is often seen as having a form of ADHD.
How much of this show is just happening in Dexter's head? The constant Reset Button could be an indication of this.
Is Dexter actually the smarter sibling, or just the more motivated one? There are plenty of episodes that indicate Dee-Dee actually understands all of Dexter's technology (sometimes better than Dexter does), she just doesn't have the inclination to put her knowledge to use.
The part of "Continuum of Cartoon Fools" where Dexter holds a stopwatch while periodically saying 'BAAAAWT'. Those who have ever did timing scripts for animation dialogue know exactly what he's doing. Word of God says that he was timing a storyboard of the cartoon itself - yet indeed, there was no other reason to put it besides the fact that it was funny.
The end of "Sun, Surf, and Science" in which after spending the entire episode competing for Dee Dee's affection, Mandark is saved from the surf by the surfer dude she was seeing - next thing you know, they stare longily at each other and walk off into the sunset arm-in-arm.
A lot of the aesops that Dexter earned involved problems that were started by Dee Dee being a Jerkass, such as in "Filet of Soul".
In a certain episode, Dexter finds an old abandoned part of his lab that he has built but has forgotten about. His old robots turn against him, when Dee Dee arrives just in time to save him and show him one of his first inventions. Dexter couldn't care less and just wants to run away to safety, then Dee Dee gives him a nice speech about how his Fatal Flaw is making too much progress and not caring about his old inventions at all... that's not how science works.
Broken Base: Over the 3rd and 4th seasons. Some fans think they're completely terrible, while others agree but consider a few episodes to be pretty good.
Cargo Ship: Dexter really loves his computer. And the feeling's mutual.
Creator's Pet: Mandark and his parents became this in seasons 3 and 4, which is considered yet another symptom of the show's Seasonal Rot.
Pairing up Dexter and Blossom is surprisingly popular.
And in a lesser extent, Dexter and Buttercup - which, interestingly, is actually supported to a small degree by canon: in a vocal announcement back in the late 90's/early 2000's in during one of the credits, it was announced that Dexter has a crush on Buttercup, because she's sassy. There were even hints in a few Cartoon Network games, such as in the Cartoon Network online game "Cartoon Cartoons Summer Resort", where Buttercup pointed out the feeling was mutual, and the two danced at the disco. And their interactions aren't even Belligerent Sexual Tension either, as others would suspect.
Mandark gets this treatment a lot. It helps that he's just more of an ass than an actual villain.
Dexter is a somewhat mild case of this. Although Dee Dee is generally the antagonist, there are several episodes in which Dexter is genuinely being a jerk to her when she doesn't really deserve it, verbally abusing her when her worst crime is being nosy. Some viewers tend to forget that both siblings are capable of being jerks to each other and that Dexter is not 100% innocent.
The shorts "Dial M for Monkey" and "The Justice Friends" are just as popular if not more so than the main Dexter shorts.
Within the main cartoon itself, Dexter's Mom and Lisa the babysitter. Guesswhy.
The Touchy-Feely Neighbor Lady is widely remembered, despite only appearing in one episode.
Mandark's abrasive younger sister and Dee Dee's rival-turned-friend Lalavava only appeared in one episode, yet has a substantial amount of fanart, enough to have gained her own category in some fandom groups for the show.
Lee Lee, one of Dee Dee's friends, is one of these
Douglas Mordecai, one of Dexter's friends, despite being a borderline Satellite Character in canon. Maybe just because he was the only character Dexter could talk to like a human being.
Val Halen has a ton of fangirls.
The Puppet Pals. Some people wished they had their own Spin-Off.
Family-Unfriendly Aesop: As mentioned under Broken Aesop above, Dee Dee once taught Dexter that he should pay more attention to his old creations and not be so preoccupied with new stuff. Seems pretty good... if it weren't for the fact that science is about making progress.
Fridge Logic: In "Photo Finish" when Dexter infiltrates a photo processing place to retrieve photos of his lab and comes across the alphabetical storage room, he checks every single section rather than just the one corresponding with his last name (whatever that is). Though it's possible he did that just in case the photos had been misplaced.
Genius Bonus: A lot of gags on the show are science references, some of which are fairly obscure.
In one of the DC Comics stories, Dexter finds a comic book based off of his own life, however the last page shows that somehow in the comic, he died. Dexter's first voice actress, Christine Cavanaugh, would pass away in 2014.
Mandark's parents are already bad enough for denying him his interest in science, but he's clearly uncomfortable with his birth name of "Susan" and how they try to overly feminize him. For LGBT viewers, it's a little uncomfortable seeing a little kid have both a lifestyle and gender role forced on them that they don't feel suits them. Mandark's rage at Dexter laughing his name is understandable because not only is it cruel, but Mandark himself even hates the name!
Val Hallen is blonde, based on a Swedish god, wears black, and plays lead guitar. Skwisgaar Skwigelf from Metalocalypse is blonde, Swedish, can't find his biological father (leading him to believe he's part god), wears black, and plays lead guitar. Cue WMG that Val Hallen's his father.
In the Mexican dub of the pilot episode, when Dexter is turned into a rabbit, he adds the line"What's up, doc?" Years later, Dexter first actor from the dub becomes the current voice of Bugs Bunny.
Part of Dexter's plan to get Lisa the babysitter to fall in love with him is to turn himself into a teenager. He does so, but ends up a homely and grossly stereotypical dweeb, driving her away. Later, Cartoon Network would create FusionFall, where many of the younger characters seem to be teenagers, at the oldest. In the game, Dexter's older self is... not a gross dweeb. Farfromthat, in fact.
Idiot Plot: The episode "Sis-Tem Error" has Dee Dee accidentally shutting off the lab's power and spending the rest of the episode tricking Dexter into thinking nothing is wrong by disguising herself badly as Computer, Robot and Mandark.
Incest Subtext: Between Dee Dee and Dexter in several episodes, though mostly on Dee Dee's part. Particularly in one episode where she's pretty much infatuated with him and even flirts with him!
Dee Dee: Hey, Dexter! Whatcha doin'?
Dee Dee: [Resting her arms on Dexter's head] Mmmmm, I just love to watch you work.
Rewatch Bonus: Ego Trip features a clever little detail that hints at the ending. The robots that come back to the present don't attack Dexter or try to fight him even as he destroys them. The first time through, it just seems like Dexter was too fast for them. The second time you realize it's because Dexter built them and they weren't there to attack him in the first place. Dexter's egotistical belief that he had to be the one who saved the future started the whole plot.
The Scrappy: Oh, Koosalagoopagoop undeniably. Pray his antics charm you, otherwise the episode just got a whole lot more annoying.
Seasonal Rot: The two seasons made without Genndy Tartakovsky after the show was Un-Cancelled due to lackluster plots (with slow pacing), Flanderization, and a new more bland-looking art style. Plus, they got rid of the Justice Friends and Dial M for Monkey segments. Oh, let's not forget that the two final seasons are about everything, butthe laboratory mentioned in the title. Most storylines of the new episodes involve Dexter playing chess, Dad playing Golf, DeeDee dying Dexter's hair blonde, and so on. In fact, the final episodes of the show are about: a) Dexter and Dee Dee learning Karate; b) Dexter writing poetry; c) Dee Dee going nuts over an ostrich.
So Okay, It's Average: Seems to be the general consensus behind the infamous "Dexter's Rude Removal" episode now that it's finally been released. Not really as offensive as expected with the amount of hype, and just generally meh.
For those who don't outright hate the post-movie seasons, they're usually considered this. They're humorous and some of the episodes are more memorable than others, but overall, it's nothing special compared to the rest of the show.
Some fans think the Un-Cancelled episodes had some genuinely neat ideas (like a backstory for Mandark, more focus episodes for Dexter's parents, the return of "Go, Dexter's Family Go!", etc.) that were bogged down by lackluster writing and animation.
And then there's the belief that Dial "M" for "Monkey" and The Justice Friends were strong enough concepts for their own show.
Most fans agree that the Un-Cancelled seasons used hideous character designs.
Invoked with "Creepy Eyed Girl" and her Spear Counterpart. They have giant eyes that look more like a Precious Moments character than Dexter characters.
Unintentionally Sympathetic: The numerous times Dexter is depicted as a Jerkass for yelling at Dee Dee donít hold up due to the Made Out to Be a Jerkass usually being in play. Not to mention that when she isnít destroying his lab, a lot of problems are cause because of her merciless teasing of him.
Dee Dee whenever Dexter destroys something of hers for example in Down in The Dumps. The total disregard she shows for Dexterís things and even Dexter at times makes it hard to feel sorry for her, especially since a lot of those instances were in retaliation for something she did. Her punishment in "Sdrawkcab" is one of the best examples. She makes Dexter puke his lunch out, get in trouble with his mother and even sent her own father to the hospital. Her only reaction to being called out on the awful things she's done is to just stay there and smile like an idiot. If not even dangerously harming her dad makes her feel bad in the slightest, then she might as well cross Moral Event Horizon.
Unpopular Popular Character: Koosalagoopagoop is not well liked in the Land of Koos, but Dee Dee, the audience, and eventually even Dexter like him.
Dexter, for those that don't see him as The Chew Toy. And to a few that do. He's especially Woobie-ish in "Old Man Dexter" when his family excludes him from movie night because he's not old enough to stay up late.
Mandark at the beginning of "Mandarker". This was before Mandark became the show's borderline Big Bad, so it counts.
Dee Dee, sometimes. It's implied that the reason she keeps going into Dexter's laboratory and annoying him is because she wants to spend time with her brother. Many younger siblings could relate to her.
Word of Dante: The infamous "Rude Removal" was made for the personal amusement of the show's animation staff as a way to let off steam from the grind of having to work on an animated children's series; it was never intended to be a regular episode of the series proper, and nor was it ever intended to be released to the public... until a few years back. Because of the subject matter, many fans keep insisting that it's a banned episode from the series, even though it was never included in the series, and as such, was never banned.