And yes, I experienced death quite a few times. My first playthrough was with a Wizard, which can be both male and female, and my initial fumbling of her abilities caused death several times - although by the end I was deftly juggling her abilities and surviving very well. The enemies have that simple-but-effective method of layered attack: swarmers surround you while lieutenants back them up and ranged attackers sling from the back. The Wizard was able to fend them off in several ways: by firing out a mirror image of herself that absorbs damage for a while and taunts enemies to attack it, tossing out frost novas which have a long cooldown but actually chill enemies to the core (complete with icicles hanging from their outstretched arms), unleashing arcane orbs that exploded with color, and using a powerful magical melee-style attack that hit multiple monsters. All of it felt right and played fantastically, with great feedback for almost every attack in the game and superb visual cues for showing what's happening to opponents without having to include buff bars and the like. And no, the developers don't plan on allowing mod-makers to design new interfaces like we've seen in WoW, and I suspect it's because people will start adding bars and buttons that Diablo simply doesn't need.

The Barbarian charged into battle much more readily, relying mostly in this demo on multiple-monster-hitting Cleaves to take enemies apart. He doesn't have mana - instead, he has Fury which works a little like World of Warcraft warriors do in that hitting monsters increases it. But the developers wanted a simpler visual style, so he increases his Fury to fill up three orbs, and different attacks will get bonuses based on how many orbs you have filled up. You won't ever be unable to unleash attacks, though, so don't worry about having to wait for your Fury to fill up just to fire out something nasty - the orbs just mean that you'll hit harder with your special attacks.