Diablo 3 - Encounters

Blizzard has updated the Diablo III site with information on the enemy encounters:

One of our primary objectives for Diablo III is to make encounters with the grotesque beasts and demons of the Diablo universe even more tactically interesting than they've been in previous games. The challenge lies in doing that without removing the accessibility, speed, and forward momentum that the series is known for. Here's how we plan to accomplish that goal. [...]

For example, when you first see the unnerving Vessels -- human cultists who've given their bodies over to demonic possession -- you'll notice that they stand numbly in place, tapping their staves on the ground and seemingly ignoring you. Yet that doesn't mean that you can ignore them. Within an indefinite amount of time, the monstrosities possessing the Vessels will tear through the flesh of their living hosts and emerge onto the battlefield, utterly consumed by rage and hell-bent on inflicting grievous harm. When encountering untransformed Vessels, it's in your best interest to kill them as fast as possible -- but trying to accomplish this can put you in the center of a melee, or take your focus off of other targets who'll savage you in the meantime. It may not seem very heroic to kill first and ask questions later, but the fate of Sanctuary is at stake - and demon-slaying is not for the faint of heart.