My 2 cents

What is the quickest Que'ing roll in WOW? Tank right!

Why? Because alot of people are not comfortable/ or have never tried tanking.

I personally feel Blizz giving Blood the buffs is a great encouragment to other players to start tanking ( DK's start off in the 50's, have early acccess to blues, and have a pretty easy ride to 85 if they choose tank - Instant Que's)

And who knows, maybe the guy who tried tanking doesnt like the DK. But has now tried tanking instances and has some more confidence to try tanking as a druid/pail/warrior.

I know people will flame and say "tanking isnt hard" but thats not my point. Like healing there isnt another role like it in instances ( unless you count melee dps??) And of course loving all the Elitest's, who dont want to take the time to let a new tank practice or leave after the first wipe or tell him hes the "worst tank ever". Which for me anyways, discourages them from tanking again...

or maybe im just a crazy Pali!!!
My personal experiences and feelings are as follows:

Tanking PUGs(Since I'm not apart of a clan for it and I need good tanking PVE gear gems before I start.) is !@#$ing horrible. The people are typically nasty and/or mutes who rarely say anything outside of "go". And as you said in your posts no one is willing to let you learn because Blizzard never created a tutorial for it. It's a thankless job and even if you do keep people from wiping they will still spit on you because "you are a bad". Your armor gets wrecked to !@#$ and the gold is equally %^-*. I made the majority of my money from mining not PVE. You are also expected to know the instance inside and out.

Really, tanking is horrible because the expectations are high, but the rewards are low. Blizzard is way too inconsistent with its incentive structure. The only reason I plan to tank is simply for because there is some gear I want. But I can probably avoid even that because Blizzzard rehashes a lot of the same gear models.

Blood may be great for tanking,( I am told it's not), but for pugs anyway it's just not worth it to put up with the people. The low rewards are also a massive turnoff.
Exactly, there is no " Learning Curve" for new people who want to tank.

I feel that making DK's not only easy to level but also have the ability to be good tanks now, will help encourage people to try the roll themselves.
I use to be a dps, and hated it. So I decided to go Tank, yeah hard at first. But then I started getting more appropriate gear, I done this with the help of a guild. Now I find myself in the same situation in cata with my DK. If you need help I'd be more than glad to run you through heroics and you may have almost any tank gear that drops, unless of course I actually need it. How is this possible? Because Blizzard was kind enough to make it possible to invite cross realms, just pst me in game and ill hook you up with my real id. And if you have questions I'm more than happy to help!
Tanked a Well of Eternity last night, with half my gear being dps gear. (tanking gear was gemmed and enchanted)

easiest tank ever hot damn
Tanked on a paladin for 4 years. I gave it up because Tanking is an annoying thankless job.
But, not the point here, there is not "training" for tanks. Starter area quest to teach tricks like LoS and CC would be of great help. I had to explain to an 85 "tank" in a heroic What "LoS" was and how to do it.

Players are leveling too fast to learn the basics.