What's a Panzerkin?
A panzerkin is exactly what the name says -- a moonkin-spec'd druid who can tank. Put another way, panzerkin are about balancing out two competing drives, damage per second (DPS) vs. damage mitigation. Though often panzerkin only perform as an off-tank, picking up those mobs that stray from the group's main tank before they hit the true squishies (mages, warlocks -- even priests to a certain degree, though they don't wilt like real clothies when attacked), in most instances, they make quite capable main tanks.
Why can moonkin tank?
So why not call them Abramskin? Well, the World War II panzers were known in conjunction with blitzkriegs, or "lightning war". Moonkin have the damage mitigation to tank fairly well, but have a much more difficult time keeping up their damage per second (DPS), if only because they run out of mana. There's no way to beat Wrath-spamming your opponent for DPS, except that, eventually, without a ton of Elune's Touches and a great shadowpriest, you'll run out of the mana to conjure Wrath up. This takes some good gameplay skills, as well-timed mana potions become especially important.
Clothing a Kin:
You may have heard the term "clothkin." These seem less prevalent in The Burning Crusade because there's a lot more +spell damage leather gear, but clothkins are those who sacrifice armor to pile on cloth equipment. The bonuses of cloth gear targets the classes that usually wear it, mages & warlocks, and have a great deal more +spell damage for Wrath, making DPS go through the roof.
This is great when you're soloing, and can use roots for crowd control, and helps a balance druid fill a DPS slot in a group, but loses two-thirds of the bonuses the moonkin form gives -- extra attack power and the armor multiplier. I'd also suggest that "of Nature's Wrath" gear, which I'll blog more on later, is a better way to pick up some more DPS and damage mitigation than cloth gear for very little gold.
Striking a good balance between DPS and damage mitigation gear is pretty difficult. I've found I tend to wear nearly pure feral gear while panzering, with just a few extra pieces to add DPS to my Wrath.
Why the Armor Bonus if not Tanking?
Taken another way, let's answer the question, "Why does moonkin have as much armor as bear?" from the Blizzard forums. From the OP:
There is just no way that a caster form should have the same amount of armor bonus as a tanking form. Ever. Period.
Perhaps because moonkin is a tanking form? From the first reply...
In a nutshell, the Moonkin armor level is to give you a measure of survivability if you pull aggro from trash in an instance - allowing you a limited ability to offtank.
The way I see it, they got a bump in armor in order to promote some measure of scalability now that the game has extended 10 additional levels. Playing a heavy hitting caster that neither has an aggro dump, nor any aggro reduction talents is a wee bit dangerous. Armor helps them.
The point of these two posts is certainly true... As I've said a few times, being a druid means you've been the main healer quite a bit, like it or not. I've healed long enough to know that when a tank loses aggro and a mob rushes a clothie, the health bars drop like the bottom fell out. It's a real pain healing mages who draw aggro before they go down and the chance for wiping begins.
But healing standard DPS moonkin who draw aggro? No problem. If a moonkin can't finish off one loose mob, well, they aren't worth their Deeprock Salt. Even then, the healer has a lot larger window for timing the heal than with a mage. The armor is a good bonus, even when just DPSing.
But, as I've already suggested, it would seem there's another message from Blizzard. They could have left moonkin at 360% for their armor multiplier after the bear nerf, and nobody would have complained. At 400%, matching bear armor for armor, the message seems to be, "You can both tank, fools!"
(I'll try to edit this post a bit later, but thought I'd get this much out now.)