My problem with Luffy is this.
Imagine he wasn't the protagonist, but another One piece character was. This is a show about someone who wants to become pirate king. Of everyone in this universe, who dyou think would be the most likely cadidate to reach that postion.
Luffy may seem like the obvios choise simply because he defeats the toughest bad guys and while doing so gets to spew all the hokey monologues about friendship and streght, and how bad the enemy is. But none of that is part of Luffy's natural character. When you take that away, what is Luffy is not a immature, overambitious, loud-mouthed, not very bright kid; a characature of a brat.
Is this such a person really the most convincing candidate for a protagonist about someone who becomes the most successful person on the sea. Zoro, or Ace, or Sabo, are all way more qualified to be protagonists if the show had chosen, and they would make more convicing commanders/ captains as well.
But of course One Piece is supposed to be a comedy, so hence it's okay to have a protagonist like Luffy who is meant to be laughed at half the time.
The problem is, when the show decides to become pure drama, with intelligent subplots and charachter developments, Luffy just comes out looking like what he really is...a kid.