It seems to me that expertise is the least discussed tanking stat for druids. Not only that, but it's often simply dismissed as a DPS or threat-stat. My approach to tanking differs quite a bit on this.
As I started researching my future as a feral druid, it became obvious that armour was the key to druid tanking. Mitigation is the most reliable way of reducing the damage you take, it smoothens your damage profile and makes sure your healers don't hate you(something that has always been at the back of my mind having my self been a healer for most of TBC). Beyond a smooth damage profile, it seemed stamina was the best scaling stat, indeed nothing increases TTL quite as much as stamina. But inflating your healthpool is only good as long as your healers are overhealing, and even then you best be aware that you do not do so at the cost of higher spikes of damage. At the end of T7 raiding I've settled into the groove of about 40k health(raid buffed), opting not to stack stamina from gems, gear nor enchants. Quite simply, what I get naturally has been enough, and I still see gearing for other things as being more beneficial to my tanking overall.
So then what am I left with? Most people would assume I then stack agility for high amounts of dodge, but as a former healer I never much liked the damage profiles of avoidance-tanks. Instead I've picked quite a few pieces of gear for expertise instead of agility and stamina. It's the stat I value the highest, and the reason is simple. As agility is both a threat and dps stat(ap and crit) and a avoidance(dodge) and slight mitigation(armour) stat, so is expertise a powerful offensive as well as defensive stat. Just as the golden rule for DPS is that if you don't land attacks, you lose more damage than if you stack a bit more AP or crit, so you lose more threat from getting parried than you would gain from the bit more AP and crit, especially with frontloaded threat abilities such as swipe now is. This means for good threat, stacking hit and expertise is key. But whilst hit only reduces incoming damage based on applications of infected wounds(if you've opted to take the talent, as I have), expertise reduces parry chance, and this in my opinion is the second best source of mitigation for a druid tank. Put simply, each parry has been shown to increase on average the speed at which the next attack comes in by 40%. That's nasty. That's spiky. That's potentially deadly and at the very least, it increases incoming damage by a truckload. This is all counter to my philosophy of having a stable and smooth damage profile. I'm currently sitting at 11.75% reduction to parry chance, and I'm intent on switching out a few gems to take it up to 12% or more. Early druid gear had the luxury of having a lot of expertise available, and the base 10 expertise we can get from talents helps us greatly. And there's nothing I love more than hearing my healers remark on how little damage I take and how steady it is over time. Simply put, do your healers a favour, don't forget to stack some expertise as well.