This short animation is very well-done and accomplished plenty of it right.

1. Snowdrop, the filly named in the title, is extremely well-written. Unlike characters in the canon, she is blind, foreshadowed by her opaque pupils, dialogue, and monologue. It's a unique physical flaw that helps her stand out. However, the story never says it so outwardly, a big strength. For one, it focuses less on her disability and more on what could she can do to make her worthy. She never uses it as an excuse. (In the animation, she's a blank flank like the Cutie Mark Crusaders.) Secondly, it gives room in the script for Snowdrop to be treated as an equal by some parts of society, notably her teacher (Mrs. Windith), mother (Primrose), and princesses.
   
2. Because of her blindness, her other senses are highlighted, such as touching and hearing. It's said to be a myth that a blind person's other senses are more enhanced due to blindness, but those who are blind tend to pay more attention to them compared to those who can see. The flashback with Primrose foreshadows this.
   
3. The monologue of her on the cloud alone shows her pain and anger. Throughout her time in the classroom, she is teased and mocked by her classmates, because they claim that her blindness has hindered in her growth. She's happy and safe with her mother, but in school, she feels lonely and ostracized. Because of this, she works solo and tries to hide her anger. The bullying she suffered in her classroom resulted in her losing some of her trust towards Primrose, a common side effect in bullying.
   
4. The origin of the snowflake is extremely believable given the folklore canon we currently have. Equestria is a fantasy fairy tale and uses a lot of mythology and folklore, true or made-up, to correlate with the universe. In the canon, winter is very gentle and welcoming, but in the real world, winter is a mixture of happiness and sorrowing overcast skies. The origin here mixes the two, but in a way that objectively works. Snowflakes are gentle, fragile, and a pleasure to see and analyze. Snowdrop has a fragile conscience, blind, and didn't know where to go in life. But she has the ability to judge the other three seasons without the need to hold prejudice towards winter. Spring, summer, and fall are celebrated, but winter is shunned; Snowdrop wants winter celebrated, too, and her snowflake is the key to making it memorable.
   
5. The lighting. The color scheme and strokes are in character to the canon, but there's great usage of the lighting to give the anatomy of the characters some depth, examples including young Celestia's tail and the bodies of some of the characters. In the scene where the teacher's class faces the princesses, the shadows are behind them, forcing the viewer to focus on the front layer.
   
6. The musical score fits perfectly with the animation. It's soft and gentle like the snowflakes themselves, and they enhanced the mood of each scene. Without the score, Snowdrop wouldn't be as strong as it currently is.
   
7. The reason why she was chosen is believable and in character of the princesses. Her snowflake wasn't well-shaped, and it wasn't pretty. However, because of her disability, she knew and analyzed things others couldn't. She has another perspective on life because she's blind and wants to spread her gift and eventual lore to future generations. She sensed the snow's shape and used her feather to create something that the princesses could understand and enjoy.
   
8. Luna's eventual friendship with Snowdrop is hinted here. During Snowdrop's last monologue, Luna looked back to recollect. During the intro of the pilot, Luna was described as feeling jealous, sad, and angry that her night was discriminated by the citizens of Equestria. Snowdrop is blind, so she can't visually judge the sun and moon, the day and night. In Snowdrop's point of view, both day and night are equal. This is refreshing for Luna. But the fact that she didn't get the chance to say good-bye after Snowdrop passed away was something that hit her hard. It's tough to say, "good-bye" by itself, but it's difficult without getting the chance to do so personally.
   
9. At the beginning of each winter, one of Snowdrop's original snowflakes is used. It's a great, touching tribute and is a method of commemorating her legacy. Without her influence, winter wasn't celebrated. The fact that Celestia (and Luna following her return) kept her original snowflake until the very end shows how much she meant to them and especially Luna.
   
10. The last line before the credits is very powerful, touching, and a respectful gesture to Snowdrop's legacy long after her death. Celestia and Luna used all of her snowflakes, but always kept the original to honor her. One thing can make a difference, and her snowflake (plus the room it's kept in) shows prestigious love, honor, and respect the princesses had for her. But even with her final snowflake dumped, Snowdrop's legacy will always be remembered by them.