Experience From Kills
 

Like almost every other MMO, killing an enemy will grant you experience.  However, the greater the level difference between you and the monster you are killing, the less xp you will receive. That is to say, if you have a level 10 barbarian, killing a level 5 or level 15 enemy, both kills will result in less experience when compared to killing, say, a level 10 enemy.  This is similar to World of Warcraft, wherein  killing lower level mobs will grant you less experience, eventually diminishing to zero.  However, in World of Warcraft, killing enemies higher than you actually carries a substantial xp bonus. I’m guessing that Blizzard has decided to penalize these higher level kills in Diablo 3 as a new way of discouraging some of the power leveling that went on in Diablo 2 (remember the Cow King?)
 
It will be interesting to see if this system stays in place, or if Blizzard simply resorts to the same system used in WoW, where xp gains are higher for killing higher level mobs, but when in a group with a higher level character the xp gains are penalized accordingly.  I guess we will have to wait and see what the final outcome is.