We changed the way armor worked because we thought it was a bad wow gold design that a druid with the Naxx 25 armor trinkets would be amazing and one lacking them would be far behind, perhaps to the extent that you wouldn't be invited to tank. As a few people have pointed out, the change to the armor calculation is as much of a nerf as not getting those trinkets to drop is a nerf.

Having armor as a good stat is fun. Having armor as such a good stat that you don't really care what else the item has on it is a problem. (And to be fair, I initially didn't realize the situation was quite as broken as it was, so thanks to all the persistent druids out there on this and other forums.) While some of you liked the element of having to get a couple of fairly rare drops to be a really good tank, other druids seem to understand the problem and agree with the change.

Stuff besides armor
The other part of the design is that without bonus armor being so ridiculously good for druids, you are left with fewer stats on which you can try to improve your tanking survivability. This is the "It may have been broken, but it's all we had" argument.

While I think it is a bit silly to totally discount Stamina ("because nobody wants to be a mana sponge") and Dodge ("because it has diminishing returns") as bad tank stats, overall we are sympathetic to the idea that it's fun to get drops that make you a better tank and druids don't get to do that as much as other classes. We're talking about other ways to scale your mitigation through gear. I don't have any announcements to make about how that would work, but you should keep a couple of things in mind: 1) We wouldn't want to do it through armor or dodge, since making wow gold easier to get those values very high unsolves the problem again, 2) Adding additional mitigation of any kind would probably mean having to nerf Ferals in other ways (like say armor) to keep you from getting too far ahead of other tanks. But we need to see what block actually does for paladins and warriors too. (Source)