Not sure if stress testers had access to the beta forums or not, but there was a post not too long ago that said Nightmare and Hell difficulty will ramp up obscenely, and that crowd control will be essential to defeating them.
Easier said than done. Crowd control comes at the expense of damage dealt, and damage not being dealt is time being wasted. Diablo is not an MMO and they'll be infuriating a lot of people if they turn boss runs into raid-like affairs that are impossible to solo and that take hours to complete.
Diablo has been/should always be about killing gobs of monsters in quick fashion.