His charm, his melodies, his suffering and his romantic approach.
He writes usually very "charming", just listen to some of his pieces and you know what I mean. This is probably also the way Chopin played himself.
He is a master of melodies. He can composer many beautiful melodies, and not only melodies, but also the way he connects them. What Rachmaninoff (I mean the guy at the forum, not the real composer
)said, every time the main theme comes back it's a little different, so usually there are no unnecessary repetitions, and it "flows" well. But he doesn't always begin easier than the end. Take the FI for example. That's a very hard piece to begin with.
Many of his pieces contain some elements of suffering, melancholy. That makes them beautiful, IMO.
His writing is very romantic. Many people can play Chopin as they want
to, with freedom and rubato. That doesn't mean it always sounds good with a lot of rubato, but people can feel free playing Chopin. And many people love using pedal, which is present in Chopin's works (and they overuse it in Chopin
That's about my way of thinking
. It's hard to describe, the best answer you can find is in his music. Just listen and play him and you know what it makes it so great.