Phenomenal character development, and people who argue that the pacing is slow, it is so intentionally, amd it helps the narrative. It's a show which will impact you, making you rethink how you go on with your life, probably even make you a better person. The first season is more of action-focused and the second is different, focusing more on the character progression. A must watch for someone bored of the typical shonen animes.
Plus, the first anime I have seen where you can notice visible ageing on characters, really impressed by it.