The entire plot of One Piece seems to be based around this. If you fight Luffy without having hurt one of his friends, you at least have the possibility of victory. If you hurt one of his group... not so much.
Massively subverted in one fight with Kuma. The man made the Straw Hat pirates vanish one by one, and Luffy couldn't do anything about it.
And if you hurt one of YOUR group members, you are so screwed. Boa Hancock & sisters, we're looking at you.
Also, let's not forget Captain Kuro, whom Luffy defeated in righteous anger for turning on his own crew.
And Caesar Clown, who gleefully sacrifices his own Mooks for the sake of his experiments. He got constantly punched to the face and finished with an Armament-haki Bazooka for his trouble.
Never hurt people who shown hospitality to you. He won't hesitate to try and kill you, even if you're his own crewmate. If only he gave Zoro the opportunity to explain why he fought the people of Whiskey Peak...
Do NOT say you're the one who is gonna be Pirate King within Luffy's earshot. (Not as mockery. He at least tolerates the ones he sees as rivals)
As of the timeskip, he's also not terribly thrilled with people who harm big brothers
And somehow, Hody Jones thought it was a good idea to threaten Luffy's friends, Threaten the Kraken's younger brothers, and claim to be pirate king. He's lucky Luffy just gave him a royal ass kicking
While played more for comedy, taking his meat is a fine way to piss him off.
A new way to piss him off is to mock his brother Ace's murder. So long, Don Chinjao.
Also, Buggy the Clown. If you insult his clown nose, he will freak out, and given the chance, he will shoot you with a cannonball capable of destroying several city blocks. When you account for the fact that he tends to mishear things and assume they were insults to his nose...
Robin is also quite angered by those who disrespect history , as she demonstrate on a Mook and Yama in the Skypiea arc. She also gets angry when people talk about her past and why she got her bounty often because what they're saying is not true.
You better not treat life lightly, especially if you're a doctor, or else Chopper will beat the hell out of you. He also lost his shit when he found out that Caesar Clown was experimenting on children and kept them in captivity with drugs.
Usopp is a Lovable Coward, and would rather run away than fight. But if you insult Luffy in front of him, you're doomed; he may be the weakest of the Straw Hats, but he's still a One-Man Army when he needs to be.
Sanji will beat anyone who mistreats the ladies to a bloody pulp, especially if the lady in question is his dear Miss Nami. Also, he often goes batshit when called weak or something like that. And if you don't like his cooking or his table service and vocalize it? You're so dead.
And don't let him catch you wasting food either; that's a big one. That includes using food to fight with. He and Old Man Zeff almost died of starvation when Sanji was a child, so this one's very understandable.
Zoro is normally the type of guy who doesn't take revenge on things. But once, Ohm's traps brutally cut and mangled Chopper...
He destroyed Kaku after being at best evenly matched throughout their fight after Kaku started insulting Robin. "You ran your stupid mouth too much."
Do not spend money without consulting Nami first. Do not steal anything from her, especially if she stole it first. And above all else, do not hurt the people she cares about.
Recently, we get one for Boa Hancock: You so much as lay a hand on Luffy, and not even a Logia Devil Fruit Power will save you from the Snake Princess tearing your ass to shreds. Yeah, when Sengoku mentioned that Hancock is strong? That is NO Informed Ability, she's the strongest Action Girl in the whole world.
If you so much as injure any of Shanks' friends (such as Luffy), expect Shanks to show no mercy.
Portgas D. Ace can be seen as a deconstruction of this trope because in the Marineford arc, when he was rescued from execution, he let his emotions get the better of him. When Akainu starts to insult Whitebeard, Ace is drawn into a hopeless battle that ultimately kills him. This happened despite everyone, including Whitebeard, warning him to run away.
Hurting Franky's Family is ill-advised. And when you talk bad about Franky's adoptive father Tom or take his speedo, prepare for the worst.
Brook's afro is precious to him as it is the last defining feature he has that his old friend Laboon can recognize him by. So ... Do. Not. TOUCH. THE. AFRO.
As of chapter 645, Brook hates people who waste lives. And considering his past, this is understandable.
In One Piece Film: Strong World when man-eating ants ignore Brook because he was considered "leftovers", he angrily states it was "a bit inconsiderate" and proceeds to cut all the ants down. This Berserk Button is pushed again in Chapter 682, when the little dragon from Punk Hazard looked disappointed that Brook didn't have any meat on his bones. This actually serves as a minor plot point, as it reveals that Kinemon also hates dragons, but for a much darker reason.
Brook also doesn't take the idea of his True Companions being attacked, or potentially killed, very well (again, understandable, due to his past).
Smoker cares deeply about his subordinates, even if he doesn't show it. Just try and lay a hand on them. Vergo, we're lookin' at you.
Bartolomeo, a character who first appeared in the Dressrosa arc, was introduced as a rude, Ax-CrazyJerkass. It's bad enough if you provoke him at all, but he absolutely worships Luffy; say one bad word against him in this fanboy's hearing, and it will be the last thing you ever say.
Kelly Funk from the Dressrosa arc, a boxer short in stature and temper with prodigious strength but no endurance at all. Call him shrimp or anything of the sort, and he'll pummel you within an inch of your life. As for the "no endurance" part that makes him less than a threat to those who are at least as strong as he, his Devil Fruit powers let him merge with his brother, a Gentle Giant with a ridiculously strong and sturdy body. When that happens, the threat level skyrockets.