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.

Experience Rewards

Experience is often given as a quest reward, in Diablo III. In previous games experience came only from killing monsters, plus the huge bonus from the Ancients quest in Diablo 2. In Diablo 3 lots of quest rewards, at least in the early going (which is all anyone outside of Blizzard has gotten to test yet), grant an item and something like +7500 experience. In many early quests, the experience reward is worth many times more than the experience gained from killing the monsters required to finish the quest, making these rewards an essential part of leveling up a character.

Maximum Character Level

During most of Diablo III's development, the max level was said to be 99 or 100. The D3 Team seemed set on repeating how that was handled in Diablo II.

That changed during development in 2010, and in September of that year the developers announced that level 60 would be the highest level possible to attain in the initial release version of the game. This is the same as World of Warcraft allowed in the initial version, before adding 10 to the level cap in each of the first two expansions, and 5 in the third. It seems a fairly safe bet that the Diablo III team has something similar in mind for their planned expansions.

The explanation of the level 60 change was quite lengthy; here's a partial quote. [1]

The 60 level cap we’re working with came from a lot of time and thinking about our content and how we want the experience to feel at every level. It so happens that it works for us really well.

Of course it all comes down to an XP curve. We could, for instance, say the level cap in Diablo III is 60 and then pace that curve and gain out over what we estimate it took someone to reach 99 in Diablo II. Of course we wouldn’t do that but it should help illustrate that the time from 1-60 in Diablo II does not equal the amount of time it will take to reach 60 in Diablo III. 60 levels versus 99 levels doesn’t mean less content or less powerful characters, etc. These aren’t uniform levels of power that move from game to game. And in fact we are pushing a longer game than Diablo II and I’d argue our characters feel way more powerful.

The leveling experience is always going to stop somewhere because the real game is the item hunt. So, instead of letting it drag out to a less meaningful 80 or so levels like most people saw in Diablo II we have 60 levels of awesome; at every level you’ll get a meaningful and noticeable increase in power. It has a ton of other benefits and fixes a lot of problems a higher cap causes.

The design theory behind this change (besides allowing them to include a higher level cap as a feature in expansions), is that characters gained very little from the last 10 or 15 or 20 levels in Diablo II. A character was basically done with their growth by 80 or 85, and levels after that took a long time to achieve and didn't provide much character benefit or change. Lowering the max to 60 allows the developers to make every level up feel important and valuable.

The theory on character progression is something like 30/50/60, when ending Normal/Nightmare/Hell difficulty. This means the highest level will be achievable without endlessly repeating areas. The developers are also promising extra content, beyond simply replaying earlier areas.

Bashiok: We also have types of content that Diablo II did not have. That’ll remain mysterious for now.

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 long series of kills. The player is informed of this bonus by text popping 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 [2], 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.