Hunger

According to Jeb on Twitter, Hunger will be added as a new aspect of gameplay to Minecraft.

If you have 90%-100% food you will regain health slowly, but if you have 0% you will lose health instead. On easy the player's health would slowly drop to 5 hearts, 0.5 hearts on normal, and all the way to death on hard.

While the player is sprinting, the hunger bar will deplete faster than when not sprinting.

This game mechanic would allow food to be stacked, increasing the amount of food one can carry in their inventory at a time. It has been hinted that this will be added when Hardcore mode is added. Despite that fact, some community modders have already created Single Player mods and Multiplayer mods, both of which incorporate the hunger mechanic.

Bugs
Sometimes the hunger bar will stop depleting. This may be due to a glitch with the new food items.