Personally I love the idea. I understand if a lot of people will be a bit disappointed, though -- the net effect will most likely push druids off the pedestal as the best EH tanks.
It'll help the following:
-Magic damage. I see no reason not to assume the shield will work on magic, as druids currently have the worst magic defenses. Paladins have ~.86 reduction, warriors have ~.85, druids have .88. Both paladins and warriors have additional benefits, though... AD for paladins and Spell Reflection for warriors. Death knights have, what, .85 in Frost Presence, Anti-Magic Shell and Icebound Fortitude? Plus at least Unholy has some more I think (Magic Suppression, Anti-Magic Zone, Bone Shield?). I'm not very into DKs yet.
-Removing PvP gear from the top EH lists. Crit, hit, Expertise will increase this ability, and PvP gear has very little of this.
-Threat translates into mitigation.
That's just what I can think of off the top of my head.