Experience

Experience is earned by killing monsters and finishing quests. Nothing is yet known of the formulas used in Diablo III, but it's assumed that the process will work roughly as it did in Diablo II.

Gaining Experience

Killing monsters of about the same level as your character will gain your character more experience, while monsters are worth a decreasing percentage of their full value as they get further above or below your character's level. It's likely that experience will be gained from quests as well, similar to how it works in World of WarCraft.

Play testers from the BlizzCon 2008 demo build (October) reported that the level up speed, from 5-10, was about the same as it is in Diablo II, or perhaps even a bit faster. How well the progress rate in that demo of the game will correspond to the final game is not known.

Level Up Bonus

Level up display.

Characters gain 1 skill point and 5 attribute points (which are automatically distributed) when they level up in Diablo III. Other bonuses are added as well, including gains in mana, life, damage, to/hit, and more. Details are not yet known (or finalized, with the game still in development).

The actual level up moment is displayed very vividly in Diablo III. Instead of just a sound effect, there's a bright visual graphic, as a funnel of orange light washes down over the player, exploding outwards. Any monsters in range are blasted by the level up bonus, which deals considerable damage.

Experience Combo Bonuses

A new feature in Diablo III grants experience bonuses for multiple kills in rapid succession. Characters score the usual experience for each kill, and then get a bonus on top of that for any series of kills. This bonus pops up on the screen whenever a character mows down five or more monsters in a row. In the BlizzCon 2008 build, characters were allowed a few seconds between kills to keep the bonus counting. For instance, if there were a dozen zombies on the screen, at least one needed to go down every 3 or 4 seconds for the bonus to keep adding up. Once there were no kills made for about 5 seconds, the bonus would display in big flashing letters and numbers.

Multiple bonuses could be wracked up in succession; if the aforementioned screen of a dozen zombies yielded 6 quick kills, a pause, and then the other 6, two six-kill combos would be scored. Those would be worth less than a single 12-kill combo, though.

Players who tested this feature in the BlizzCon 2008 demo noted that the experience bonuses were not massive, but they did seem noticeable. It's hard to judge such things the first time playing the game, when no one is sure how much experience anything is worth, or how long it takes to level up, so more thorough testing will be required in the future.

In an interview from BlizzCon 2008 [1], Jay Wilson said there was a maximum hard cap for the experience bonus, but that he'd set it so high he didn't think anyone would ever achieve it. Secret Cow Level style mobs would be required?

Q: Is there any caps on the Killing Spree combo?

A: Yes, there is a maximum combination combo. It’s related to your level but I don’t think anyone can actually get it. It has a theoretical limit, but you are not likely to reach it.

Jay also said that there would be other bonuses (of unspecified types) to be had in Diablo III, and that they're going to look into how to make the bonuses more equitable for slower-killing character types.