I'm rabid for just about any details of Diablo 3. You may or may not have seen these already, but recently some information was disseminated by Blizzard via its forums.
The entirety of my life, circa 2010 (or whenever Blizzard releases the game)
First, there is going to be a new death mechanic. In previous Diablo iterations, you were kicked back to town when you died and that was that. The Diablo team has realized this takes away from the action of game and the town was always a safety net for the player. Instead, they plan on using a checkpoint system, where when you die you restart at the checkpoint (possibly with a death penalty). Sounds interesting.
Also, monsters will behave more like, well, monsters. Thus far, the monsters of Diablo all seemed to work in perfect harmony to kill you the player. None of them ever fought with, or even did indirect damage to, each other. It appears that they're going to address this, though monsters won't naturally attack one another without reason.
Lastly, they are working on items in the inventory and rehauling its UI. The UI (of anything) is one of those things I typically have to see to truly appreciate, but this is freaking Blizzard. By definition, whatever they do is correct.
Death Mechanic article: http://hellforge.gameriot.com/blogs/Gaming-Rampage/Die-die-die/
Monsters and Inventory article: http://hellforge.gameriot.com/blogs/Hellforge/Bashiok-on-Monsters-and-Inventory/