Diablo's action RPG core has remained the same, and will do so in the third version. But it sounds like they are also introducing a number of new systems that borrow from WoW, Guild Wars, etc. Can someone list out the new systems with a brief comparison of how they worked in earlier versions of Diablo?
closed as too localized by Raven Dreamer♦ Jun 4 '12 at 3:10
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You can find a good official overview here. Some highlights include:
Skill system - No more skill tree with assigning points to different skills. Instead it will be more like Guild Wars. Leveling up will make skills become available, but you only have a certain number of slots for skills (6 active, 3 passive once you've unlocked them all). Skills can be swapped in and out easily, so no need for re-specs.
Rune system - Runes are variations of skills that not only give them bonuses, but can actually completely change the way the skill works. They might change a damage over time into a direct damage, or add area of effect, etc.
Auction House - There will be two auction houses supported by Battle.net. One will be gold based, the other will be real money based. The real money AH will allow carrying a positive balance, so you can sell your items for credit and then use it to buy other items without ever spending real money.
PvP Arena - PvP will be separated from the PvE game. There will be 1v1 and team competitions, and will use a matchmaking system. PvP will earn you achievements, but not actually boost your character.
Artisans - Artisans are a way of crafting new items. They will be similar to gambling in earlier versions of Diablo, where the input will be gold and items, but you will have a bit more control out of what comes out of it.
This site has the most comprehensive list.
I am in no way trying to steal this answer, I just would like to add some more information becasue I just wasted my time answering a question that closed.