Responding quickly (he says, as he begins to type - so, we shall see)...
The FREEDOM!!
Even when I was young, and playing very old games on machines that many, today, would laugh at, my favorite thing was veering off the path (even just driving on the grass in Pole Position, lol).
This game is almost entirely off of any rails, whatsoever, and I fully appreciate and enjoy that. Couldn't be much more pirate, in that regard.
The SAILING!!! (especially combined with the freedom)
Both the sailing mechanics and that DAMN BEAUTIFUL SEA. The wind, the raising and turning of the sails, the waves, the storms... PLUS the teamwork for making the galleon cruise like a dream!
The COORDINATION!! It deserves its own category here. I love working with 3 other crew-mates that jump on the sails and the anchor with me. Getting to learn that, in the beginning, was fantastic fun. And it's always fun teaching new pirates "the ropes".
I think that's a fantastic element of the game and it's a big part of drawing me to the galleon, over my usual soloing gamestyle.
Last but far from least - THE ATMOSPHERE.
This game is atmospheric. When I'm not playing, I like to turn on certain streamers, just to continue to enjoy breathing in the salty winds, feeling the splash on my face... watching the skies and the waves, ever changing. The skull clouds and the skeletons and the sharks... The game world is beautiful AND immersive.
Those are the top things that I immediately think of.
They all, also, ring true for what pirates are all about. Freedom, sailing, coordination, atmosphere.
Combine all of that with the inherent humour of this game... and, yeah, 'tis my favorite.