Right - here is my two cents.
I play a Death Knight Tank. I have no idea what tanking would be like on other classes, but I can tell you what it is to tank on my Death Knight.
You're going to have to deal with quite a bit of stigma - people have generally bad ideas about Death Knight's, they tend to think that they suck, or they can't tank, or even do anything - but let me tell you, they're wrong. I main tank for my guild in raids, and I do quite well!
Tanking as a Death Knight is certainly interesting - you have to remember to keep Death and Decay up, and Rune Strike everytime it's available - I cannot speak for Frost or Unholy tanks, but I'm a Blood Tank and find that aggro-management is very easy - Heart Strike affects two targets and I find that spamming that (when I'm not in need of a self-heal) I am able to hold aggro even when one of the DPS is attacking the wrong one! (i.e. not my primary target)..
As a Blood Tank, one of the things that appeals to me is the self-heals I get in the form of Death Strike, Rune Tap and Bloodworms. It makes the stress of being a healer alot less. I personally use my self-heals IN PLACE of a Shield.
Take the stat - Block.
It means that you Block the attack with your shield, reducing the damage taken or avoiding the damage altogether. A Blood Death Knight, will take the full damage, but will heal alot of it aswell - essentially reduces incoming damage, or avoiding it altogether, acting as a shield.
As I said - I main tank raids, and healers find me a very easy Tank to heal. Even for a Death Knight, which are notoriously hard to heal - but geared well, specced well and played well - a Death Knight Tank is as good, if not better then any of the other classes - we're good verus Single Targets and AoE.
Downside? - Our major AoE tanking ability is a placement-cast, you must click it, and then click where you want to cast it. For me, that makes it so I can't just click and forget, I have to click my cast. ><
Also you find that you can take alot of damage, due to your inability to block, and it's very easy to get so worked up on holding aggro, that you forget to self-heal and the healer panics. But if you can manage your self-heals and your aggro, it should be a very fun class to play!
And yes, we Death Knight's are popular, and there are alot of us - but how many of us are actually confident tanks that can tank well? Not many. Consider this.
Also consider that in real life tanks do not have shields. So why must a game's? They have plate armour, like a Death Knight does.