People keep saying too that Zoro is simply a muscle head and commands no real authority on the ship. And even when I offer valid examples like Zoro offering himself to Kuma to protect Luffy's dream which is what a real number 2 would do, I keep getting disregarded.
So I figure my opinion is like this:
In One Piece we deal with situation after situation where the lives of Strawhats are compromised by strong, scary, evil pirates. Given that this is the real situation, this is their reality, it makes sense that the person who commands the most power is the strongest. That is Luffy.
But it also makes sense that Zoro is the second in command under that pretense of immediate danger following the strawhats on the high seas.
I have to agree with Sereques, I don't think the types of behaviors the other Strawhats reflect that grim life and death reality that Zoro and Luffy repeatedly face because that's what THEY CAN DO. (Sanji too but he's the cook and that has been explicitly stated over and over and over and was totally confirmed when Zoro knocks Sanji out as Sanji tries to take this responsibility from Zoro)
Even Sanji himself in Enies Lobby recognizes this fact and conveys to Ussop his belief that each crew member is good for what they're good for.
In Zoro's case, Zoro is the protector of the crew. When Luffy is finished, or Luffy cannot help the other Strawhats, you can bet that Zoro will be there guaranteeing their safety. Just look at in Punk Hazard when Nami, Robin and Chopper were about to be killed by Monet. Zoro was there to protect them.
I think the other Strawhats recognize this trait of Zoro and respect his power on the ship accordingly. There's a reason Zoro mostly trains, lifts weights and in general doesn't participate to the crew in the happy go lucky way Ussop, Franky, Brook, Chopper and even Luffy do. Zoro recognizes his innate responsibility to be there for his crew when they need them. But this goes far beyond combat scenarios.
Just because they are the best examples doesn't mean we cannot simply deny that they happened, and Zoro telling Luffy how to handle Ussop was the perfect example of Zoro flexing this authority he commands. Luffy didn't want to listen to Zoro, but did because he understands Zoro's position in the Strawhats and changed his course of action because Zoro brought up.
Yes, Sanji did confirm what Zoro said, but it wasn't Sanji's place to say that. He could only agree with Zoro.
And then once again in Punk Hazard we see Zoro flexing his authority to Luffy when he told Luffy to get his stuff together. This direct challenge just simply doesn't happen with other Stawhats. They do not have to fill that role and cannot fill that role.