Ambiguous Disorder: Lizzie has strong skills with puzzles, riddles, and numbers, has frequent panic attacks, has stunted social skills, only has one friend who is just as odd as she is, and doesn't have much physical prowess. Ripred immediately senses that she is "different" from others. Seems like a high functioning degree of autism, or Asperger's Syndrome, but the series doesn't go into details.
Fan Preferred Pairing: Even though the canon pairing is Gregor/Luxa, many fans seem to prefer spoiler:Gregor/Howard for some reason.
Or Gregor/Henry. Or Bane/Ares. Or Ripred/everyone.
He's Just Hiding: Many fans refuse to believe that Twitchtip is dead, mostly because Gregor (and the audience) only found out about her "off screen" death through a bit of intercepted information. Some say she faked her death and is out running in the Underland somewhere while others say the rats were just lying to each other.
What Do You Mean, It's for Kids?: Yeah, it's still a book series aimed at older children, even though it contains a massive ammount of extreme violence such as decapitation. Wounds are even sometimes described in explicit detail. Maybe the fact that it's written media and not shown as actual pictures keeps the Moral Guardians away.