Blizzard had a press event for Diablo III back in July that was kept quiet until the NDA expired on August 1st. It's expired and here's tons of info that came in.
First up, skill trees are out. If you're still reading and not passed out, it appears that Blizzard has decided to make things simple in Diablo III. Players will have skill slots available(start off with 2, goes up to 6) to insert whatever skills they want from a pool of skills. Skills are unlocked as you progress through the game, and it appears, at least for now, you can switch them at anytime. Traits, like skill slots, can be switched out with a pool of traits that get unlocked throughout the game as well. In both cases, it lets players build their characters without any consequences or need for a respec.
Diablo III will be online only. Obviously this is Blizzard's attempt to keep their game from being pirated, and like other games that have done this before, it's going to start a shitstorm. Maybe not as much as the removal of the skill trees, but a storm nonetheless.
Finally, and to me the most interesting, there will be auction houses that let players buy and sell items, gold, and possibly characters for real money. Similar to Ebay, Blizzard will charge for listing fees, transaction fees after the sale, and a fee on cashing out the money. I have to say hats off to Blizzard for this. They've realized that why bother trying to protect your assets left and right to shut down people making money off your product. Instead, let people do it through your game and take a piece of the auction. Not just that, it appears they're taking a cue from Zynga by letting people that want to pay to get ahead to go right ahead and do so. It's like if the U.S. government legalized drugs and taxed it. I'm all for it since I don't do it yet there's plenty of people with the money that would be willing to.
A little gameplay video was also made public to get a taste of how the game is going to play.
Source - Diablo III Database