I just read this thread cuz I pondered picking up my druid tank which I haven played since tbc, wait why couldnt bears tank kara? I tanked it just fine on all bosses, I was a lot less squishy than our warrior. So back then when there was maintank and offtank, I was always maintank. I dont remember any fights at all where I felt squishy or in anyway the one slowing the raids down. And this was about mid expansion before bt lvl upgrades from heroics, since I started again mid tbc.
Been bear tanking since 2007, and feel stronger now than I did in any previous expansion. The only fight I feel gimped on is Thok and that's only because I have a warrior co-tank whose Shield Barriers make him untouchable. Also because his Shield Slams will hit for like 1.5M non-crits, and make me look at my 800k Mangle crits in shame. But other than those two situations on that specific fight, bears kick ass and don't die.
I do wish Displacer Beast could be used by bears without putting us in cat form, though. It's a super useful and fun talent but losing 100% of rage and vengeance to blink 20 yards seems like an unfair price compared to all the other specs.