Cursed, Inc interview from Blizzcon 2013 with Travis Day and Jason Regier:
From around the 1m05s mark…. Travis Day basically responds to how soul binding will work:
The way that soul binding is going to work in Reaper of Souls is that, we’re basically reserving it for the best things in the game; only for the legendary items, the set items. Players will still be able to freely trade rare items that drop and everything below that.
