The question says it all, what changed from Diablo II to III?
closed as not constructive by Wikwocket, Sorean, Raven Dreamer May 15 '12 at 19:22
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This site has the most comprehensive list.
Here are the largest changes:
Loot is now personal, meaning all of the gold and items that drop are exclusive to you. So nobody can come through and snatch up your loot.
You no longer place your skill points as you level. Stats are distributed based on your class and skills are unlocked as you go. You can constantly be changing your build to optimize situations and PvP.
Runes are now skill enhancers and can no longer be placed in weapon sockets to gain bonuses or create runewords.
There is a fourth difficulty called Inferno which is built to put your maxed out character to the ultimate test. It is very tough and has been said to be two times harder than the agreed difficulty the testers thought was sufficient.
Trading is now done through the auction house (much like WoW). There is an AH that you can use real money to purchase items and also a gold based AH. NOTE: You can sell items in the real money AH and gain real money without investing a dime into the system.
There are now Artisans, which are NPCs used to craft. You can salvage items much like you can in other RPGs and create gear. The gear created usually won't be better than what you can find naturally looting and slaying.
Those are the core changes, please refer to the site for a more comprehensive list. I will work on this a bit more when I get home but I have to get back to work. :)