some more details from the last hour of the live stream:
focus on co-operative play
focus on replayability ("different experience each time you play through" or something)
world that feels alive
large scale battles
potions no longer a core part of gameplay (lead dev talking about previous hack 'n' slash games said "when you come across a monster thats really hard, you just pound the potion button til you win" - they're trying to steer d3 away from this)
haven't decided on inclusion of hardcore yet but "we probably will" (wtf, damn right you will)
mythos-style loot system (when grouping, any loot you see is yours and is invisible to other people, or is at least allocated to you and can't be picked up by them)
new battlenet has vastly improved cheat protection
oh and fl00tz: just as i was typing this a guy asked if the witch doctor was the new necro, and the answer was something about the necro being so damn badass they don't see how they can possibly improve it
edit: presentation over. only other thing mentioned was aesthetic character customization, and the last two questions were completely ****ing wasted with system specs ("we want it to run on as many systems as possible" - same answer from every dev since time began) and map editor ("we're not really focussing on mod capability")