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.
Then there's the scene during "Master of Disguise" where Robbie sneaks up on Stingy, a 6 year old boy, while singing "You walk right by me, not knowing that I'm slimy!" complete with a creepy predator-esque expression on his face.
According to this thread, Robbie Rotten is a Tragic Hero, as the series actually takes place After the End and his goal to keep the kids inside is actually to protect them from radiation. Sort of Jossed in "Sportacus Save The Toys" when Sportacus visits France and China. Both countries seem to be fine.
The fandom is known for its interpretations of character backstories. Fanfiction writers often give Robbie Rotten a tragic past. A few diehards have created interpretations of the mythical "Number Nine", mentioned only in one episode. A lot of fan theories revolve around Nine being Sportacus' father.
This article argues that Robbie Rotten is the Only Sane Man and that trying to let children play and eat sweets is actually preferable to Sportacus' enforcement of brutal physical torture and diet of seeds.
From the original play, ╔g ß Gˇan Vin (I Have A Good Friend) was very heartwarming.
In the episode "LazyTown's Surprise Santa", Robbie Rotten snuck into the kids' houses to steal their presents and sabotage Christmas dinner because he thought he wasn't invited to their Christmas party, only to discover a present under the Christmas tree with his name on it and an extra seat at the dinner table for him. Daaaaw!
There's also the episode "Dear Diary", where Robbie stole Stephanie's diary with the intention of writing nasty things about Stephanie's friends in there and leaving it somewhere for her friends to find. But as he was flicking through the pages, he noticed all the nice things Stephanie had written about him. He then changed his mind and gave her diary back, intact.
In "The Lazy Genie", the titular genie accidentally makes Robbie disappear. Sportacus saves the genie's lamp, so he is given one wish. He gives the wish to Stingy, and what does he do? He wishes Robbie back.
The original Icelandic shows gave us the extremely catchy MegabŠt.
Ensemble Dark Horse: Robbie Rotten is considered the funniest character by many. The fact the show dedicates quite a bit of time and effort to his plans and props (more than to any other character, even Sportacus) supports this.
Ho Yay: There is so much Sportacus/Robbie slash out there that it seems to outnumber any other kind. And it doesn't come from nowhere either: one episode had Robbie falling asleep and happily murmur "Hello, Sportacus."
Memetic Mutation: "You are a Pirate" and "Cooking by the Book" (and its Lil' Jon mix). Also, Stephanie herself used to be a major internet meme on 4chan, where she was usually photoshopped to be shown... working out with a Pedo Bear.
As well as, within the "You are a Pirate" song, a line about "precious booty."
Julianna Rose Mauriello (Stephanie's actress) turned 18 in 2009; the internet as a whole had much rejoicing, if all of the posts of "She's legal now!" are to be believed.
And to an extent, Robbie Rotten and his *ahem* package.
Then there's Stephanie, who is supposed to be eight, but her male fans only see the teenage actress playing her.
Periphery Demographic: A lot of moms and teen siblings are as fond of the show as the kids it's aimed at, due in no small part to the 'talents' of the human actors (yes, all three).
Several years ago, a portion of the Starlight Express fan community became a bit of a periphery demographic for the original LatibŠr stage shows.
Portmanteau Couple Name: The fandom has come up with portmanteau names for the show's two most popular pairings: SportaSteph for Sportacus/Stephanie, and SportaRobbie for Sportacus/Robbie.
Self-Fanservice: Oh, the things the fandom does to Sportacus and Robbie Rotten... The extreme athleticism of the former and... suspicious bulge of the latter may be bordering on Parent Service. Either way, the Periphery Demographic has made some... interesting things of both of them. And, from an entirely different quarter, Stephanie.