The first class I tried out was the Witch Doctor. It's a caster class with voodoo- and necromancer-style magic. My main ranged attack was Poison Dart, which did some damage over time to monsters I hit. Adding a rune, it changed to a fire dart, which did more up-front damage at the expense of the DoT. I also had a skill that summoned pets to help me fight -- up to three Zombie Dogs at one time. They were actually powerful enough that I eventually let them die so I would have more to do. Runes affected them differently — one rune gave them a chance to drop a health orb when they died. Others modified how they did damage. The AI was pretty smart about having them attack the groups I wanted them to, and not go charging off on their own.
The skill I chose when I hit level 10 was Firebats. It functions similarly to the Inferno spell in Diablo 2, only instead of projecting a cone of flame in front of me, it projected a cone of flaming bats. The first Skill Rune I looked at extended the spell's range, trading off damage to do so. That was a bit underwhelming, since I already had ranged attacks, so I went with a different rune that turned the cone into a whirlwind of flaming bats that surrounded me. Another rune added a life-drain effect. It was tempting, but the whirlwind looked too cool to pass up. Finally, the skill that sold me on the Skill Rune system was Zombie Charger, a spell that summoned a zombie, who would shamble a few steps forward and then spray poison in the immediate vicinity. It was a solid, short-range attack. Fortunately, I lucked into a rune that modified the spell to summon a group of zombie bears that charged forward and trampled whatever was in their path. The buddy I was playing with happened to get a rune at the same time that made his Wizard's basic lightning spell change into a massive, conical spray of electricity, and we couldn't help but laugh as we used our new-found power to demolish groups of monsters.
The next class I tried was the recently revealed Demon Hunter. The class mixes ranges weapons with gadgets and dark magic for its offensive power. It bears similarities to both the Amazon and the Assassin from Diablo 2, but feels distinctly different to play. My main nuke, when I started out, was called Entangling Shot. It would do some damage and slow the monsters it hit, and then chain to other nearby monsters. Using Entangling Shot, it was quite easy to control entire groups of monsters. This was supplemented by a skill called Bola Shot, which would send a glowing bola out to wrap itself around the neck of a monster, exploding a few seconds later. It supplemented the slow effect of Entangling Shot nicely; by alternating between the two skills, I could drop big damage while keeping anything from reaching me. The Demon Hunter also had Vault, a movement skill that makes you leap forward through the shadows. It's a good way to get range — perhaps too good.
When I reached level 10, there were some interesting choices for new skills — a Fan of Knives provided area-of-effect damage in a circle around the Demon Hunter; Molten Arrow sends a blast of fire through monsters; Multi-Shot sends a conical rain of arrows that looks more like something out of 300 than the equivalent Amazon skill in Diablo 2. But I ended up choosing Grenades. The Demon Hunter tosses three grenades at the feet of her target, and they explode for huge damage. The cool thing about this skill is that you can bounce the grenades off walls, so you can blow up things around corners or in the back of a group that's rushing you. Unfortunately, I didn't get to try any runes with these skills. Overall, the Demon Hunter felt a lot more dynamic and... percussive than an Amazon.
I got a chance to tinker around with the Barbarian as well. This class uses a different resource system — fury is generated by certain attacks, and is then spent on other (generally more powerful) attacks. It certainly played differently than the range classes. Wading into groups and Cleaving away was standard operating procedure. I enjoyed this class less than the others, mainly due to the fact that fury generation seemed fairly slow, and my skills weren't that focused on damage. One ability, Revenge, was reactive, only becoming available occasionally during a pitched battle. At level 10, I picked up a skill that made him leap into a group of enemies, dealing damage when he landed. The trouble was that I didn't feel like I really had a go-to attack. I'd guess that this would be solved by different skill selection, but obviously the demo build had its limits.