Don Krieg fight is overall kinda boring and overly drawn out for a character who really isn't that interesting, HOWEVER I do love what it establishes about Luffy so early in the story. Of course there's the consistent pattern that he fights for his friends, for the weak and oppressed, and against heartless tyrants. Watching Luffy do that never gets old. But the way the fight goes down also reveals some of Luffy's specific brand of crazy: he's completely unbothered by pain. He views it as nothing but a temporary obstacle, and sometimes even a totally acceptable path to victory. It doesn't make him stop and rethink things, and that drives powerful people CRAZY.
Don Krieg shoots harpoon-like bullets into him, and Luffy just pulls them out and fights him bleeding. Don Krieg shields himself with a cape made of spikes, and Luffy just punches it anyway even though they pierce his knuckles. As Zeff points out, he has no fear of death. But even more than that, Luffy refuses to be told he can't.
Every villain and tyrant in the story tells him some version of "it's impossible, you can't do this without destroying yourself, you'll never survive, this is just the way things are," and Luffy comes back every time unafraid to destroy himself, determined to survive anyway, and absolutely unwilling to accept that things have to stay bad. The villains always bet on some sense of self-preservation kicking in, but Luffy doesn't have that the way other people do. He outsources his self-preservation to his friends, and fights for them with reckless abandon, and people in power cannot fathom that kind of communal trust. He will never do things the way they think, and that's why he'll always win in the end.
