I just made an in-dept TL;DR post about this, but managed to eraze it all before posting, so I will cut to the case and keep this "short".
Bears get Savage Defense to compensate for our lack of avoidance. Savage Defense is the reason Bears scale insanely well, and in a very non-linear manner. The problem with this unique tank-mechanic is that it is quite inferior to avoidance in the beginning of an expansion where stats are generally "low" compared to a later stage of the expansion.
The reason Savage Defence scale so well is because it is greatly based on synergy from other stats. More hp = more ap, more agi = more ap and crit %, more ap = higher absorption values, more crit/haste (even white hit-rating if you want to be picky) = more Savage Defence procs. This means that essentially, our mitigation/avoidance is based upon stam, agi, dodge, ap, crit, haste, (hit). Warrior/Paladin, as an example, only rely on block, parry and dodge for mitigation/avoidance. This leads to Warriors/Paladins having an easier time obtaining decent survivability, than Bears.
Blizzard nerfed Savage Defence which resulted in less mitigated damage. Then there was made a decision that Bear HP-pools were to high, and our HP was nerfed. Savage Defence was nerfed even more along with it (because of the AP we lose when our HP-pool shrinks).
I've tanked HCs non stop for 3 weeks now (both with all out *stat*->dodge reforging and agi-geming/enchanting, all out stam-gemming/enchanting *stat*->dodge reforging, and hybrid gems for socket bonus and a balance between agi and stam) and wanted to spice it up a little bit, so I went Resto OS yesterday. My discoveries were quite shocking. After healing an equally geared Warrior I noticed how much Bear HP actually "spike" in comparison. One of the reasons for this is obviously that Savage Defence is pretty bad versus multiple targets, but the difference were so huge that I had to ask him about his actual values. He had 47% parry/block? and something like 7-20% on each of the two remaining stats, if I remember correctly. This is more than 30% more avoidance than I got with my not so awesome 32% dodge. Naturally this will save the healers a lot of mana and, as far as I know, healers running OOM is usually what causes wipes at this stage of the game. You do the math..
Is all of this intended? Do I fail to see something essential in my analysis?
Turned out this post didn't end up as short as I intended, but I hope people will take their time to read it and join in on the "debate".