Hunger

According to Jeb on Twitter, Hunger will be added as a new aspect of gameplay to Minecraft. If you have 90% food you will regain health slowly, but if you have 0% you will lose health instead. 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.

As of 19.07.11 Notch posted pic of what could be the hunger bar on his twitter. This could be related to the Realistic difficulty, and may be upcoming in 1.8 update.