What I like about One Piece is that every fight is justified. Battles don't occur just because Luffy wants to be the Pirate King. There is always another, more important reason, usually to correct injustices made to the inhabitants of an island, to one of his nakama, or both. If One Piece fights were all about who's stronger, I wouldn't have watched. That's Kaidou's way of thinking. There's even a reason why Luffy wants to get stronger: to protect his nakama, so that he'll never have to lose any of them.
I like One Piece because it has a bit of everything: comedy, suspense, mysteries, sad past stories, values of friendship and true justice (unlike the government's so-called justice), correction of wrongs and that there is no black-and-white good vs evil, but people can change sides and help Luffy even after he defeated them, without the remains of any grudge.