Ambiguous Disorder: Dil, who is described as "Not the brightest candle on the menorah", with Phil and Lil mentioning they feel responsible for his mental condition because they dropped him on his head.
In one episode Didi states that she thinks it's genetic. "... from Stu's side of the family of course."
Believes the world is a tetrahedron, dearly hopes to learn to speak with cetaceans, junior UFOlogist, and prefers walking backwards (not just for fun; he thinks it's the clearly superior mode of locomotion).
And it was revealed in an episode that he was literally dropped on his head when he was little. By Phil and Lil.
Though it's left ambiguous whether or not that's the cause. Didi seems to think it just comes from Stu's side of the family.
But it doesn't matter, because his family loves him and appreciates his unique ideas whether it's true or not.
Crouching Moron, Hidden Badass: Chuckie, when it comes to obscure skills and talents necessary to get his friends out of unusual, dire situations. To his dismay, none of these skills would be useful at any other time.
Darker and Edgier: The Christmas episode "The Finster Who Stole Christmas" has a darker tone then the rest of the series, with multiple references to God (even Lil referring to him), sins, and stealing.
The two-parter about the camp with the disappeared pilgrims' curse.
Dark Is Not Evil: 'Z'. He is a dark, badass, punk, volunteer, helping to raise money for inner city schools.
Even Evil Has Standards: Inverted in "Lucky 13". Savannah the Alpha Bitch deliberately schedules a party on the same day as Angelica's birthday celebration For the Evulz. The rest of the group are appalled and decide that even Angelica didn't deserve that.
Executive Meddling and Out of Order: The season 1 episode "Lucky 13" was held over from airing until August 11, 2004, which marked the 13th anniversary of when Rugrats had premiered. A few season 2 episodes premiered before "Lucky 13".
The Christmas episode also refers to God quite a few times. Children's series tend to not talk about religion.
One scene in "A Deville House Divided" has Lil completely stripping a doll naked during a play session with her friend. Luckily, you don't see anything...and then Phil introduces a robot with plungers that fire out of its nipples.
One episode has Didi catching Stu watching a hula girl dancing suggestively on the TV.
Growing Up Sucks: Often averted, ("Fear of Falling") sometimes played with, ("Wouldn't it be Nice?") or played straight, ("Coup DeVille").
Happily Married: Chas and Kira, Stu and Didi, Betty and Howard, and Drew and Charlotte.
Especially Chas and Kira. As seen during the episodes "The Finster Who Stole Christmas", "Memoirs of a Finster", "Interview With a Campfire", "Trading Places" and "Petition This". Sadly, Kira was only in 12 episodes of this series.
Informed Judaism: Averted, as Tommy and his family celebrate Hanukkah, Passover, and Tommy attends classes at a synagogue, and tries to win over girls using the "nice Jewish boy" tactic.
This is carried over from Rugrats, which had Hanukkah and Passover specials.
Incest Subtext: Phil and Lil shows shades of this. Especially in the episode when Phil accidentally walks on Lil as she was trying on her bra. After that he seems to have an instant attraction to women in general and both are uncomfortable when around each other.
Then Lil shows signs of this when Phil starts dating her best friend, seeming to want to get her brother's attention back rather than her friend's.
It's All About Me: Angelica's attitude in general. Best summed up in her response to a boy who had a crush on her.
"We come from two different worlds! Mine, and everyone else's."
Karma Houdini: In "Susie Sings the Blues", she pays $1000 to a record dealer to make her a singing sensation. Turns out she was just conned and left out in the middle of the slums. The "producer" disappears from the plot just before the reveal, and recieves absolutely no comeuppance for her actions.
Lad Ette: While Lil does try to be a girly-girl to fit in with everyone else, many episodes show that she's a Ladette at heart. Her friend Wally from the soccer team is one also.
Laser-Guided Karma: With Angelica it's not so prominent here than in Rugrats, as she's more of a protagonist than the antagonist she was there, but there's still examples of her machinations coming back to bite her with pinpoint precision. There's even an entire episode dedicated to it.
Limited Wardrobe: Played with. Adults have but one outfit each, but the kids have quite a few looks they rotate through.
My Sister Is Off Limits: Chuckie's reaction to Kimi dating. (And to his suspicion that Tommy may have a 'thing' for her.)
Phil shows signs of this when he starts to notice the gender differences between him and his sister. Lil herself shows signs of this as well when Phil starts hanging out with her best friend. Many fans take this as Incest Subtext.
Mythology Gag: Many references to Rugrats appear, with the obvious (entire episodes revolving around finding Reptar or Cynthia), to the more subtle (Tommy still keeps a toy screwdriver in his pants, albeit being more sensible to put it on a keychain in his pocket).
Nice Jewish Boy - Invoked by Tommy, who tries to use this trope to impress a girl he likes.
Noodle Incident: One episode has Lil threatening to blow the whistle on Phil doing "you know what". Subverted at the end of the episode where we find out he spray-painted his skateboard on Betty's wedding dress.
The Other Darrin: Notably averted. Everybody had the same voice actors as they did at the time on Rugrats (Joe Alaskey and Nancy Cartwright had taken over for David Doyle (dead) and Christine Cavanaugh (retired) already).
Precocious Crush: One of the rare times we really feel sorry for Angelica in "What's Love Got to do With It?"
Put on a Bus: Z. An entire episode is dedicated to him and a possible romance between himself and Kimi in the first season. By the next, all that's left is a passing mention and a brief, non speaking cameo.
Also Nicole Boscarelli, whom Chuckie attempted to impress several times in season one.
Reading Stage Directions Out Loud: Angelica tries to stage Harold desperately asking her to the dance in the hopes of making a boy jealous. It all goes south when Harold dramatically reads out "get down on bended knee!".
Shower Shy: The episode "What’s Love Got to do With It" has Chuckie trying to avoid having to shower in public, until one of his attempts to do so gets him stranded outside the locker room with no clothes or towel. At the end of the episode he finally overcomes his fear.
Spinoff Babies: Inverted; they were babies in the original show, and older in the spinoff.
Ship Tease: TP+KF has Kimi admit that she once had a crush on Tommy and the ending of the episode suggests she still is.
Token Minority: Sad but true, Susie and Kimi, though they are more than that.
Took a Level in Jerkass: Every character save for Dil and Angelica (yes, ANGELICA.) has fallen to this, either in single instances or as a group. The biggest example would be the entire group turning on Tommy in "All Broke Up."
Considering the age gap though, it's not hard to wrap your head around the fact.
Too Much Information : Apparently, when he's home alone, Phil likes to be completely clothes-less; Tommy makes a mental note about always calling the De Villes' place before visiting.
Totally Radical: The series made somewhat of an artform of apparently creating pseudo-slang that nobody would ever actually say for no apparent reason. "mondo-weirdo"
Twin Switch: Obviously Phil and Lil in "Runaround Susie."
Two-Teacher School: Played straight sometimes and averted others. Over the course of the series we see about four regular teachers plus their vice principal and some one-off elective teachers... however they also tend to reuse teachers for other classes depending on the episodes. As this is middle school, it's somewhat justified: many middle schools have the students have one main class and a few others, so them having the same teachers all day, with others only being around for electives, makes sense.
Vague Age/Wise Beyond Their Years: from All Grown Up to All Growed Up(Which, while it premiered first, takes place after the series proper): Dil is 9-10, Phil, Lil, Tommy and Kimi are 10-11, Chuckie is 11-12, and Susie and Angelica are 12-13. Each of them tend to act as though they're a few years older.
Vitriolic Best Buds: Susie and Angelica. On the one hand, they're fierce rivals - Angelica is resentful and often antagonistic towards Susie's talent, and Susie shows great disdain for Angelica's meanness - whenever something involves taking Angelica down a peg expect Susie to be on board right away (if not spearheading it) and vice versa. Over the course of the show each enacted some antagonistic plot or another against the other, but on the other hand they're very good friends who tend to look out for each other when the chips are down. In fact, they seem so used to their rivalry that their battles against one another seem like just another part of their friendship.
Chuckie assumes Kimi is going this way in the episode "Bad Kimi". He's wrong.
Wig, Dress, Accent: Lil does this when Phil is pretending to be her (and ruining her reputation). She dons a blonde wig, large sunglasses and sombrero to hide herself.
X Must Not Win: Angelica seems to encourage this sort of behavior in people. Tommy acts like this towards her and alienates his friends in "Rats Race," and in "Petition This" Susie acts like this towards her and accidentally causes a lot of grief for Kimi, who Susie was supposed to be helping. In general, Angelica is like this towards Susie succeeding where she doesn't but tends to fail; subverted in 'In The Family's Way', when Angelica starts getting the upper hand and Susie (with help from her brothers) resort to playing dirty against Angelica, resulting in Angelica winning in the end and Susie (and her brothers), for once, getting grounded.
You Would Make a Great Model: An episode has Suzie meet a woman who offers her a chance to audition to be a singer. The only catch is, she has to pay a fee for a studio session in advance. When she shows up to the "studio" it turns out just to be an office building.