Adaptation Expansion: There's a lot of this, but for starters, contrary to the original concept's solid seven, the anime went out of its way to introduce a secret eighth kingdom on the Wonder Planet, known as the Ocean Kingdom, guarded by Princess Pearl.
All There in the Manual: The character fanbooks and BIRTHDAY's original webpage gives small details to the characters that aren't revealed in the show. For example, Fine and Rein are both eight years old, Bright received Ruby on his tenth birthday, and Shade received Regina (a desert lizard) at the age of five.
Almost Kiss: Rein and Eclipse nearly share one in the manga... only to be cockblocked by Milky!
Chekhov's Gunman: Naginyo, in the first series, proves to be a stock character who just shows up at random parts in Once an Episode just to sing random songs on his harp while charming the ladies. Then, 20 episodes later, it turns out he was a messenger sent by Princess Grace THE WHOLE TIME.
Dancing Theme: The first opening theme, "Princess wa Akiramenai", had a little bit of dancing and the twin vocalists of the song, FLIP-FLAP, also did the same dance in a live-action music video version. Similarly, the first ending theme, "Oshare Fantasy", had a little dance sequence and there was a segment in one of the DVD extras that taught the dance.
Defictionalization: During the first season's run, there were actual Princess Parties held to promote the show.
Disappeared Dad: All the families in the Wonder Planet are nuclear families... except for the Moon Kingdom, where there is a queen, but not a king. In fact, we don't even know who and if Shade and Milky ever had a father...
Loser Protagonist: The majority of the princesses win Princess Parties because of Fine and Rein's help, but the two have never won one. This was averted in the manga, where they win the final Princess Party.
Oddly Named Sequel: Futago Hime Gyu!, with "gyu" being a Japanese onomatopoeia for squeezing (often used for hand squeezing or tight hugs).
Official Couple: Fine/Shade, Rein/Bright, Altezza/Auler, Elizabeta/Fango, Calorie/Tauri, Milro/Pastel... and that's not even including the kings and queens! Even Milky and Nalro got some Ship Tease before they even learned how to speak!
Power Trio/Three Amigos: Fine, Rein, and Poomo act as a trio throughout the series, although you can substitute Poomo for Lione if you want a more human counterpart. Cue the second half of the series, Altezza replaces Lione's role. The trio formation becomes crucial to Gyu! to the point where the students must create teams of three, hence Team Gemini (Fine, Rein, Chiffon), Team Celeb (Elizabeta, Sasha, Carla), Team Jewel (Altezza, Sophie, Lemon), Team Samba (Lione, Calorie, Asuri), etc.
Put on a Bus: The writers of Gyu! use this trope immensely. The show introduces new princesses and princes while ignoring even the royal families of the Wonder Planet. In other words, if you're looking for anyone who is not Fine, Rein, Poomo, Chiffon, and Bibin, prepare to be massively disappointed. Even Auler doesn't show up as much as he should have. Alongside of that, even the new princesses and princes are basically ignored after having one episode to themselves — Thomas gets this the worst, as shortly after his Heel Face Turn, he disappears for about 20 episodes, and when he finally returns to the Academy, he is Demoted to Extra.
Spell My Name with an S: Rein's name was written as "Rain" in the first eyecatch of the series, but producer Junichi Sato decided to stick with calling her "Rein" because he didn't want her to be associated with the weather rain.
Milro has an interesting case since her name was spelled "Mirlo" in the first character fanbook, but "Milro" in episode 19 and in the second character fanbook. One has to wonder if "Mirlo" was a typo or if They Just Didn't Care about the proper romanization of her name. Last, some fansubs write her name out by her literal Japanese pronunciation, "Miruro", but it's not Word of God so it has less validity than her other names.** The fansubs romanized Truth's name as "Toulouse" and Altezza's as "Altessa" before the character fanbook came out with the official romanizations. Some people still refer to the characters by their fansub names.
Sugar Bowl: The Mysterious Star is very cute and friendly.
Suspiciously Similar Substitute: In Gyu!, the vice principal of the Royal Wonder Academy resembles Chancellor Roman from the first series, from the character design to even his voice actors. The same treatment is given to Rarau and Yayan, the vice principal's assistants, who seem to be Expies of Rau and Yan, Roman's mooks, in the first series.
Swiss Army Tears: When Fine and Rein lose their hearts permanently after defeating the Black Crystal Planet, it takes Bright and Shade's Manly Tears to revive them.
Weight Woe: In the Best Sports Princess chapter in the manga, Rein becomes worried about participating due to her recent weight gain.
What Could Have Been: The writers of the show decided not to focus too much on the Love Dodecahedron between Fine, Rein, Bright, and Eclipse because they felt that romance would draw away some of their viewers.
Wonder Twin Powers: Although Fine and Rein can use the Prominence and the Feather Mewgram by themselves, they need each other to transform and use their most powerful magic. This is also prevalent in Gyu!, where their twin angel mascots, Pyupyu and Kyukyu, are needed to help them transform.