One of the most important concepts to understand in any Diablo 3 leveling guide is how experience works. Because gaining experience as quickly as possible is synonymous with fast leveling, it only makes sense to start with a basic overview of fundamentals. By understanding how the system is set up, one can ultimately work to manipulate it in order to level very quickly.
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.
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.
Experience combo bonuses is a new (and very cool) source of experience making its debut in Diablo 3. Kill combo bonuses, also called "Massacre", are awarded for killing a bunch of enemies quickly in a row. You receive your regular experience for each kill, plus an xp bonuses awarded at the end of your killing streak. The kills need to occur within only a few seconds of each other to be considered a streak, otherwise the kill counter will reset. The amount of bonus xp granted depends on the number of enemies you kill in a row. Personally, this feature really excites me because it indicates Blizzard's intention for Diablo 3 to be an all-out slaughterfest. Rewarding such non-stop mayhem definitely harkens back to Diablo's predecessors where leaving a path of destruction in your wake was not only possible but highly encouraged.
Also new in Diablo 3 is the "Pulverize" experience bonus. This bonus xp is granted for successfully springing a trap or otherwise using your environment to deal damage to a foe. As it currently exists in the beta, you're not required to actually kill the enemy with your trap to receive the bonus, only to damage them. As long as you finish the job using other means of damage dealing within a timely fashion, you will receive the pulverize bonus.
Diablo 3 offers additional experience bonuses for killing multiple enemies simultaneously with one damage dealing ability. This bonus is currently being called "Mighty Blow".
Unlike Diablo 1 and 2, quests in Diablo 3 will now award experience. From what it looks like in the beta, the experience gained from successful completion of a quest is actually pretty huge. Often it's more than the total amount of xp earned from killing all the enemies during the course of the completing the quest!
I definitely view this addition as a good thing, as it will give us players multiple ways to earn experience rather than the relentless grinding (killing many enemies for hours on end) that the previous games were famous for. Part of leveling characters quickly in Diablo 3 will be dependent on one's ability to actually stick with the process for an extended period of time, which is made quite a bit more enjoyable when you add some variety to the mix.
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