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.
Some weird demon thing lifts Alfred up from the back of an elephant in the intro. It never appears again.
In one episode, Alfred is seen building some kind of cleaning robot while he's on the phone with Winnie. Why he's building it or how he suddenly became a robotics expert is never explained, nor does the thing ever appear again.
The opening and ending themes, "Spetter Spieter Spater" and "Zo Vrolijk" ("So Happy"), as mentioned below.
"Nog Nooit" ("Never Before"), Johan's (and later Alfred's) enchanting, heartfelt love theme.
"Kijk Niet Om" ("Don't Look Back"), the awesomely catchy and intense main action theme throughout most of the series. It even gets an even more awesome variation for the last two episodes.
The ominous theme that plays during the flashback to the Evil Genie of Darkness' reign of terror, and recurs during various scenes throughout the show. That saxophone and the eerie synth makes everything feel that much more sinister.
"De Sjeik" ("The Sheikh") is a wonderfully eerie, dreamy theme.
Both the opening and end tune are still anchored in the collective memories of Dutch tv-watchers.
The tune Alfred plays on the magical violin to cure the people suffering from the Mozons virus, mostly due to repeating ad nauseum for several minutes.
Lawful Neutral: Inspector Holmes is dedicated to serving the law however he can, even if he doesn't always feel good about it. When he comes to arrest Winnie's family for being illegal immigrants, he expresses how disgusted he feels at how black ducks like them are treated, and that he would want nothing more than to release them.
Dolf starts out as a relatively harmless, slightly sympathetic school bully, and as an adult moves on to become a burglar, but it's not until he becomes the fascist ruler of all of Great Waterland and has Alfred and his friends arrested and locked up that he becomes truly irredeemable. His Evil Laugh as he watches Alfred get dragged off seals the deal that there's no turning back for him.
K. Rokodil arguably starts the series having already crossed it; first driving Alfred's family from their home just to build a theme park, and then shortly after accidentally running over his parents and siblings without looking back.
Viewer Gender Confusion: A lot of viewers were unsure whether Ollie was male or female due to his feminine voice (and who was indeed voiced by a woman). Since he eventually gets a wife and a family, we can probably assume the former.