Mythology Gag: Usagi's name translates to Rabbit of the Moon. The show does not shy away from lampshading this as much as possible, from Usagi's room full of rabbit-themed items, a Sailor Rabbit costume, etc.
Will They or Won't They?: With Mamoru. It may seem like a Foregone Conclusion if you've seen the anime, but one of the particular areas where PGSM diverges has to do with their relationship. But They Do anyway, even getting married in an extra episode.
Superpowered Evil Side: Dark Mercury is considerably more powerful than she was before her corruption, but not as powerful as Mercury after her "activation" as a Senshi shortly after returning to normal.
Anti-Villain: As with the original, the Shitennou were originally servants of Prince Endymion who were brainwashed by Queen Beryl. This is taken further than in the anime and manga; like the Senshi, they begin recovering memories of their "past lives" partway through the story, which causes them to question their devotion to Beryl.
Four Is Death: Even explicitly refer to the trope by calling them "The Shitennou."
Musical Assassin: At one point, he has a Youma sing a work of music he's written called "Requiem for a Princess," designed to harm any girls who hear it who consider themselves "princesses." Even after that, his music seems to have certain magical powers such as forcing the other Shitennou to recall their past-life memories.
Expy: Metaria is more like Chaos in the anime than the original Metaria, being more of an energy force behind evil in the world than an individual, sentient being. Unlike Chaos, though, Metaria can be defeated.
The Man Behind the Man: Though less the case than in the original; Beryl and the Shitennou seem aware of Metaria's evil and destruction potential to the point that they actually seem to be working against it by the final episodes.