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Out of all the powers in One Piece, Haki seems to be turning into an ability that is going to take Luffy, and therefore the rest of the storyline into the next "level." It has been shown time and time again to have absurdly powerful effects used by powerful people.

Offense: The most obvious. Haki can be used to do anything from unleashing an "invisible" attack that knocks weaker combatants out of the picture, strengthening attacks, manipulating objects without touching them (Rayleigh's removal of Camie's neck ring) and, arguably the most powerful and important, negate the effects that a defensive devil fruit power may have (Rayleigh harming Kizaru, Gorgan Sisters huring Luffy).

Defense: Haki users can predict their future, no other way to put it. They receive premonitions so far in advance so that if they are physically capable can respond to bad things that might have happened to them. By simply holding out their hands, both of the Gorgan Sisters were able to block the majority of the damage from ALL of Luffy's attacks that he threw at them (even Jet Bazooka only succeded in knocking Marigold back and did not physically harm her much).

Non-Combat: I don't think it's Luffy that is special in his ability to make friends with everyone. The Gorgan Sisters were all unconditionally loved by all the sisters of the Island. Rayleigh seems to have a relatively good reputation with people he meets despite a somewhat vulagar personality. Finally, Shanks has attracted such an powerful following since he became a Yonku, and it takes a certain kind of person to keep powerful people in the OP world on your side.

It seems to be a mystery why Haki wasn't explained at all or taught to Luffy at all during his encounter with a Kuja tribe. My guess is that it is being saved for a time when he needs to controls ithe super-powers it provides to defeat an Admiral-eque opponent and start making a rush through New World. It does not seem to be an ability that is a big secret, especially to more powerful and worldly characters.

My big concern with the ability is that it is going to turn a character into an unstoppable force. Luffy is still extremely likable to me due to the creativity and sheer strength he has to put into his combat style to make it work. Hopefully, Haki won't be an all or nothing deal. It is already shown that there are 2 or more levels of Haki, but the only difference I've seen so far is the ability to knock out weaker people and have a physical presence be about the character. Hopefully, Haki pans out to be an ability that includes one or more parts of the spread, but not the whole thing.