They said Diablo 3 has a complete adventure that's playable solo, and that it supports Battle.net for cooperative gameplay. I think that rules out any kind of subscription fees.
Anyway, that gameplay trailer is simply amazing. "You've seen walls of ice, fire, and bones... but have you ever seen a wall of ZOMBIES?" Animated and destructible 3D environments, monsters can fall off bridges and get crushed by walls, chests can be trapped, enemies can block attacks with shields... it's like they went through a wish list of action RPG features!
Just one thing worries me -- the environments look like they can't possibly be randomly generated. Perhaps they'll surprise us but I'd rather expect some kind of Titan Quest combination of fixed environments and random creatures & loot.