Bonus armor was altered with the release of Warlords of Draenor to make up for the removal of Dodge and Parry, becoming a tank-specific stat. As with Spirit, it’ll only show up on rings, necklaces, cloaks, and trinkets. It’s clearly valuable to all tanks, and will be tuned to be much stronger than other secondary stats. Tanks will want to use items with Bonus Armor for those slots.
It will usually appear as Bonus Armor on item tooltips.