During today’s Campfire Chat, Blizzard revealed they are increasing the difficulty of World Tier 3&4 Dungeon bosses, but at the same time, buffing the rewards you receive from them.
Dungeon bosses will always drop a Legendary item in Patch 1.1.1 from level 35 onwards.
In addition to bosses, Treasure Goblins will also have a guaranteed Legendary drop. The current drop chance is 50%, but it will be 100% in Patch 1.1.1. You want to chase and kill a goblin when you encounter one in the upcoming patch, but maybe don’t get too close with your mount!