I'd love to try a Druid tank with x4 Disc in the next expac.. I think the mastery is very promising:

Mastery: Nature's Guardian
Requires Druid (Guardian)
Requires level 78
Increases your maximum health and healing received by 16%.
Also increases your attack power by 8%.

Also you can see a bit of Disc in action here: https://www.youtube.com/watch?v=GYTCSv4FzY8