What do you think of the role-specific descriptions? I personally hope that the NPC in the harder difficulties will do all the stupid things that players do in-game, to ensure that your tank training is as thorough and realistic as possible. For example, if it could unleash all its burst cooldowns on an add just the very second it appears, while also standing in fire, that would be great.
It's excellent that the DPS version isn't just a target dummy, but requires a little active movement, dodging fire, interrupting, that sort of thing. The healing one requires similar actions to the tanking one to make the AIs as realistic as possible, and if they could possibly be programmed to say "omg no heals" when they die to big purple puddles on the floor, that would certainly add to my immersion.
Altogether, I'm really looking forward to trying out this feature, and you can rest assured that WoW Insider will keep you informed on how they all play out.