I've mentioned several times in the past what a druid is good at, tanking wise, and what they're not. In TBC druids absolutely have a niche as the best tank for certain fights, and the worst in others. This is as it should be, I believe; every tank should be able to tank anything, but be best for tanking certain things and worst for tanking others. Druids aren't there in TBC (stupid lack of shield) but they're pretty close.
And their primary niche is to be able to take more physical damage than anyone. Especially from slow, hard-hitting mobs like Gruul. This ended up not being the case as we moved into T6, mostly because druids were armor-capped so early - but they still were great at it.
Which...is being given to the warriors:
We still would like all 4 tanks to have their own niche, and unless we change something down the road, the design is still for the warrior to be the best tank for single bosses that hit hard for physical damage (which to be honest, has tended to be most of them so far).
Well, that's okay. Sure, warriors have been the best overall tanks for a while, right? More oh-shit buttons, better ability to deal with magic, fears, great threat, etc. But with DKs, they can't do the spellcasting thing as well, so they have to have something else. That's fair. But don't worry, the Blizzard devs know what they're going to be doing with druids, right?
I think the druid still needs a niche. I think "best tank when the boss can't crushing blow" and "best tank that can also dps" aren't great niches now, and perhaps weren't even before LK.
Great. So...paladins are the kings of AoE. DKs are the best at dealing with magic using mobs. Warriors are going to be the best at dealing with physical mobs. DKs are also going to be the best or close to the best at doing DPS while not tanking. What does that leave a druid? Offtanking is a possibility, but being the best at taking hurtful strikes is not a particularly fun job as a tank. Phase shifting is a possibility, but the amount of fights that have multiple phases that will also benefit from a druid tanking and then dpsing is going to be fairly low. Off-healing? Meh. Threat? Very unlikely given a bear's mechanics and itemization, especially since only one stat (agility) helps both threat and mitigation compared to strength and BV for the other classes. Avoidance is similarly being nerfed and the other tanks have similar amounts anyway.
What does that leave for druids? I don't actually mind losing the best physical mitigation crown to a warrior, but having something else would be good. What I think I'd like to see is a druid take the old warrior niche. Warriors were the best tank in general, able to do anything okay. They could deal with multiple mobs decently. They could deal with magic users well. They could deal with physical mobs well. They could deal with fear effects well. Some other classes did things better some of the time, but a warrior was good all-around with a real specialty towards magic users. Druids being able to do everything okay goes well with the theme of the class and the versatility that druids love. And it could help with progression in general, especially if you have multiple tank requirements of a certain flavor. Need two tanks that can deal with physical damage well? A warrior and a druid will work. Need two tanks that can deal with adds? A paladin and a druid could be great. Need a couple that can deal with magic users? Send in the DK and druid.
Now, all of this would require quite a bit of retooling. Druids would need magic mitigation. They'd need some kind of spell reflect or spell avoidance. More ohshit buttons that can work to stop fear effects. Less focus on threat. But it could be done.
On the flip side, it means that some guilds wouldn't want the generalist tank, because they could optimize and take the best choices all the time. It's a possibility, and it's one I don't really like. But it could happen. It's a better possibility than a druid not having any niche at all, though, and at least it's a direction to go.