There's a lot of previews out there. Here's one from Rock, Papers Shotgun.
Apparently it's a lot like Diablo. The four different characters all represent different classes with different branching skill trees, one dude is a melee guy, one is a sniper, the chick is stealthy and I can't remember what the last one does. You can play single player with just one of the four, but people can jump in Diablo-style and you can play it co-op. The classes' skills compliment each other. You can also do PvP, but you have to issue a challenge and they have to accept, if they do you get invincibility from the outside world till the duel is over. It will have an rpg style inventory system, but I don't think there are details on that yet. You'll also have a normal health bar and a regenerating shield bar.