Druid Armor is roughly equivalent to a Plate tank using Bonus Armor pieces.
The Plate tank has Dodge, Parry and Defense for avoidance
Druids have Dodge (and much less Defense) for avoidance.
Plate tanks typically have higher avoidance, while bears typically have higher health.
Against a single boss, a Druid will proc Savage Defense based on their Crit chance (40-50% in bear) t o absorb the next hit for 1500-2500 (based on Attack Power)
A Warrior or Paladin will either Block More often, or for more, depending on which class is in question. It should roughly equal out against a single boss.
Druids using Shifting Gems is fairly common for red slots. AGI effects Crit Chance (for Savage Defense) Armor, and Dodge. Druids do not gem for Dodge Rating, because AGI is tons better to socket for.