Crazy Awesome: Mukrezar starves out an underworld city by buying all the food to make the world's biggest cake, and cuts off their normal resupply routes in a way that won't trigger the mutual defense treaties.
The Fairy Sisters react this way to seeing their sister Cerasse's internal organs in the episode "Convoluted Rescue Plan, Go!" Thankfully for all those involved, the fairy was just made partially invisible instead of being horribly killed and raised as an undead horror as the fairies initially thought. A second example of characters reacting to a Nightmare Fuel situation occurs in-story in the episode "Assault on Wemos", where Keeper Mercury observes Zarekos torturing Keeper Wemos in graphic detail. The details in that scene do border on being Squick.
The priests of Crowned Death gather 8,000 civilians to sacrifice for a ritual. Male, female, young, old, they don't care. In order to prevent them from trying to escape, the prisoners all have their eyes removed.
Rooting for the Empire: Ami is something of an aversion because she's a through-and-through good girl who just happens to be stuck with Keeper powers. Mukrezar plays this one straighter.
Tiger. To some readers, her having become Ami's bratty evil twin makes her hilarious. To others, it makes her insufferable. She may be Rescued from the Scrappy Heap after we understand Tiger's motivations better.
The fairy scouts seem to get this a lot. How much time is spent focusing on just how doggedly they assume every single thing Mercury does is evil tends to get annoying. They can't really be faulted for the attitude that all Keepers are bad regardless of their intentions, but the scenes involving them seem to drag on forever. We get the point already, Pusakuronu. Good Is Dumb. Justified though.
Viewer Gender Confusion: Since Keepers can possess their minions bodies, too many viewers have had a hard time keeping track of what Keepers are what gender Keepers Alphel and Arachne both being females are two good examples. It doesn't help matters that Keepers can alter their forms. It also doesn't help matters that Mercury refers to a person's gender by what body they're inhabiting or possessing, rather than what gender they themselves identify themselves as being or were actually born as, such as when Mercury referred to herself as a male when possessing the Reapers body in the episode Mad Science for Beginners.
The Woobie: Ami. Gets transported to the world of Dungeon Keeper by pursuing Jadeite into a portal during a Sailor Senshi mission. She immediately gets roped into a Deal with the Devil situation with a Dungeon Heart, which binds her to the world in question and will send her soul directly to the Dark Gods when she eventually dies. If she didn't agree to the deal, the Horned Reaper in the dungeon would have killed her on the spot. The Light Gods want to imprison her forever to protect Ami's world from the Dark Gods' influence and all that entails. Until she finds a way to free herself from being a Keeper, she cannot go home again. And she keeps gettinginto humiliating situations. Granted, Ami is not going to let any of this stop her, but it's still going to be a long road home...