There's an interesting experiment being conducted in Wrath of the Lich King at the moment involving class balance. As Tobold pointed out the main tanking classes have all recently gotten bonuses to damage dealing in the last couple of patches. Normally the tanking classes are handicapped on damage dealing because of their high survivability. However, for some unknown reason Blizzard decided that it was imperative that they reverse this design trend. As a result warriors, paladins, and death knights are showing a remarkable ability to kill mobs in the expansion with very little risk of death. I've even noticed that these classes also seem to have the advantage in PvP at the moment. I know while leveling I've pretty much been attacked by nothing but Paladins and Death Knights.
So why did Blizzard decide to reverse a policy they've stuck with for four years and release a swarm of armor plated gankers into the game? Well, the main reason was probably the lack of tanks in the Burning Crusade. Blizzard made major improvements to paladin and druid tanking abilities, but still couldn't encourage players to specialize in that job role. It was too much of a pain to switch talents and collect two different sets of gear. Healers suffered from similar problems, but the change to the +healing stat helped them solo a little bit more effectively. Also the mobs in the Burning Crusade were tuned for classes wearing protection gear and having tanking talents. A lot of dps casters like druids, shamans, and priests could switch over to a healing role without changing out talents or gear. Yet dps plate wearers often ran into problems trying to fill in as a tank.
It looks like Blizzard has learned from the last expansion since the Wrath of the Lich King dungeons have less stringent requirements for tanking. I know my level 72 paladin was able to wear all retribution gear and having nothing but retribution talents and still tank the Nexus. In my opinion this pretty much means any plate wearing class can tank the non heroic dungeons without having to change out talents or gear. This effectively raises the pool of available tanks in the game and avoids a lot of issues the Burning Cruade ran into. However, this doesn't mean that plate wearers will remain top dog forever. Eventually, as more and more people hit level 80 the pressure to open up the new PvP season will build. Once that happens the plate wearing classes are going to stick out like a sore thumb.
I wouldn't be surprised to see that the first PvP season of the expansion goes live along with a major nerf patch. Something specifically designed to make sure tanking classes don't keep survivability along with their new found dps powers. The nerfs don't even have to be that major. I've already seen Blizzard announcing a couple in the forums and being quite honest about them. One example is that druids are scheduled for a change to how their armor class bonuses are applied to jewelry slots. It's a very small change, but the high amount of armor class on some rings in the expansion makes it important. The other tanking classes should also expect similar adjustments to any ability that raises survivability without requiring points in protection.
It's just conjecture on my part, but it seems that Blizzard wanted to encourage as many people as possible to level up a tanking class before balancing them for actual PvP. I know a lot of people hate that a PvE oriented game is balanced around a few PvP activities. However, I don't think Blizzard really has a choice. Maybe if they designed for PvP in the initial game they could be like Warhammer and have each ability work different in PvP and PvE. Now it would just be too time consuming to rework everything. Thankfully, I think the dual talent build system is almost done and I expect this to alleviate a lot of our problems. Plate classes shouldn't be able to survive a nuclear blast and keep up with a rogue on the dps meter. However, they should be able to freely change between the two.