Hardcore

Hardcore mode is a new game mode that is tied to the eponymous difficulty and set to be released in a update 1.9. The mode will be set from the world generation page. Once you generate a world with hard-core mode, you can't go back. Hardcore mode has been implemented into the 1.9 pre-release 2

When the player dies, hardcore mode only offers a button to delete the world save instead of survival mode’s or  button. The world save still exists until the button is pressed. While starting a hardcore world, the game is set to “hardcore” difficulty, which only differs from “hard” difficulty in the fact that it cannot be changed to another difficulty while playing the world. Also, if the gamemode is set on hardcore, the picture of the hearts of the life gauge change. When the menu options screen is set on peaceful hostile mobs don't spawn with the exception of cave spiders.

The idea came to Notch from a Minecraft gameplay blog on the PCGamer site.

Trivia
In the world selection screen, Hardcore mode is written in red.