Les Yay: Sun and Lunar, Sun and Akeno, Mawari and Sun & Mawari and Akeno all have moments. And the Class President tends to unwittingly have them with the other girls too, much to her dismay.
Moe: In one episode, Lunar-papa, Gozaburo, and Masa (ordered to) crossdress as schoolgirls and catgirls and act out moe stereotypes in an attempt to understand a girl's heart. It really is that horrible. Ends with Sun suddenly receiving cat ears, which everyone agrees is incredibly moe (before being blasted away).
Nightmare Fuel: The second half of the 2nd OVA, which does a Genre Shift from comedy/action to straight horror, borrowing elements from The Ring. Saturn will probably haunt your nightmares now.
The part of the TV series were Sun's family is knocked out by the creeper mermaid noble just because he wanted Sun for his own petty desires the terror on the Seto clan's faces as the special incense works to knocks them out and Ren crying out for her daughter to run away but they all wind up being knocked out.
They Wasted a Perfectly Good Plot: Nagasumi's relationships with Mawari and Lunar. They both obviously have feelings for Nagasumi, yet all the developmentnote Episodes 4,6, and 18 for Mawari, and 12 and 13 for Lunar; Lunar was also technically the first girl to confess her true feelings to Nagasumi as well. between them was never spoken of ever again after a certain point, and they reverted to how they were before said development. Which is sad really, as that would've made the plot more interesting.