Luffy has a worldwide reputation for causing trouble, challenging the Shichbukai, the Marines, and even a Yanko, committing felonies against, as well as actions which were deemed threatening by, the World Government. He has gained a reputation for being "reckless" and, in some cases, "insane", after causing an incident in Enies Lobby, entering and escaping from Impel Down, and joining the battle of Marineford.
He is, therefore, famous for being the only pirate to not only break into three most important government facilities, but also escape from all of them alive. Including the fact that he punched a Tenryubito with full knowledge of the consequences, as well as his heritage, has caused Luffy to be labeled as a "Dangerous Future Element", earning the wrath of Fleet Admiral Sakazuki, making the marines among the greatest enemies.
These acts, among other things, have given him his current bounty of $400,000,000, one of the highest past or present bounties in the series. Having had a bounty of $300,000,000 prior to his arrival at the Sabaody Archipelago, Luffy was one of eleven rookie pirates who has been referred to as "The Eleven Supernovas", the eleven pirates with bounties over $100,000,000 to have reached the Red Line.