In short: Inferno, nerfed.
Additionally, the Nephalem Valor bonus you get for killing elite packs of monsters at level 60 is being tweaked a little so that you’re guaranteed to receive one rare item each time you dispatch a group of rare or elite monsters. Bosses (that is, act bosses) will only be guaranteed to drop one rare item instead of two, so players don’t leave the game after killing one of them.
Blizzard is also removing the bonus damage that monsters receive with each additional player in the game, because it makes the game too difficult and renders certain player skills, like the Wizard’s Archon, completely worthless in cooperative games. In other words, monster damage in cooperative games is getting nerfed so players don’t die so often.