I tried, I really tried on this rewatch to find literally anything remotely close to bad or even disappointing in this special and there's nothing.

[spoilers ahead]

If I had to pick something that did bother me a bit, it's that the girl is supposedly 15 years old yet looks a lot younger (doesn't bother me much) and that at times I felt that the Admiral who found Chopper cute was a bit out of place, even though I think it's a great bit (and he barely changed any scenes). Also I guess the score had some potential to be a little better but again, not something one would notice.

The backstory of the Marine Soldier with his brother was incredible, I get chills just thinking about it. Then the special going from peak to peaker with that ending that left me in tears... unironically, for the first time in my history of watching anime I legitimetly teared up at a scene.

I loved how every straw hat was either there in the scene (Robin, Chopper, Usopp, Sanji) or the scene was about them (Franky, Brook, Zoro, Luffy, Nami), It's just so perfect.

"If I can survive... will I be able to make an impact on someone's life someday? Just like that guy."

This. Special. Is. So. Well. Written.
Every line of dialog, every shot, every interaction, every cut, is so well put together and sequenced. This is of course supported by the great director Megumi Ishitani, who famous directed beloved One Piece episode 1015. Her touch is unexplainable but you feel it when you see it. When I watched 1015 and was told it was directed by the same person who directed fan letter, it made so much sense. From the people arguing about who's the greatest swordsman (obvious parody of online discourse) to the little kids not picking up that the "Soul King" girl is distraught from the context clues, the importance of good scene writing cannot be understated.

I can't not mention the puzzle motif, connecting the smallest most unimportant side characters to the big, praised, and beloved main characters. A thing I noticed on this rewatch is that you can hear the puzzle click when the girl and the marine first lock eyes. They've seen each other before, each one saw the other but not simultaneously. It also sets this moment up as a pivotal change in the story. And all that buildup made the full puzzle connection at the end so satisfying.

Last but not least, the elephant in the room, the animation is perfect. I'm not the biggest craver (not sure that's a word) for good animation in shows I watch, and as good as the animation is in this special, the reason I'm mentioning it is more because of how incredibly fitting the animation style is to the story and the pacing. Especially the fluid movements, it really helps with showcasing the characteristics of the... characters. Additionally, the emotions shown so perfectly on the characters' faces makes this an incredble example of how to successfully perform a "show, don't tell".

Overall... what can I say, this special is up there with some of the highest highs the show has ever given me, and I will cherish it forever as one of the greatest pieces of media I have ever watched.