Experience orbs have been added to Minecraft 1.8, as shown in a trailer by Notch this afternoon. These orbs will fall when a animal is killed. When these orbs are collected, they are added to XP bar.
Notch officially showcased it live this morning at the PAX event in Seattle. The experience bar[1]
has been showcased many weeks earlier.
Notch has told us about the revamped fighting system, and this is a product of this change. He has added new features to the bows, and improved the gameplay with these new orbs