Quote from Poision Envy:
man Azmodan was such a bitch of a fight. The people that just say "Oh Diablo III is just click-to-win grindfest" has obviously never done a boss fight.
Heh, no kidding. Bosses and named/elites are the main stopping points in Nightmare for my brother and I, and for a lot of others from what we've seen. Most of the normal enemies fall pretty quickly unless we screw up or they overwhelm us, but when we run into one of several combinations of buffs such as Shielding and Vampiric, for instance, they can be downright insane. In fact most of the new buffs they add in on Nightmare are a pain to deal with.