Tanking:  Blood spec and blood presence are required.  There are no other options.  A group I was in the other night had to kick a DK tank because he was in frost presence and refused multiple requests in party chat to change.  Don’t be a fail DK tank.  Be one with the blood.

As a DK tank, your aoe threat should be enough to keep mobs off the healer, unless that healer is also dps’ing, in which case, you might be screwed.

Good luck practicing the new tanking methods in current content.  I found that in order to get aoe threat, I needed to get death and decay down, icy touch and plague strike on my main target, and pestilence to spread it around.  By then I should be far enough ahead that dps can begin.  Of course, that’s 4 gcd’s and 6 seconds, and how many dps do you know who want to wait that long to hit stuff?  What usually happened is that I would get off death and decay and icy touch, and the dps (who were at least assisting me) would kill the main mob before I could get pestilence to spread the diseases around and blood boil.  So then I would frantically try to get threat on the rest of the mobs, but they would die before I could really affect anything.
It’s frustrating to say the least.

I’m actually tempted to try some aoe tanking just using death and decay and blood boil spam and see if that’s an improvement.  At least it won’t matter when my primary target dies.  Threat may be crap, but I’m already losing threat to all of the DPS in the group.

On single mobs, I didn’t find that I had many threat problems, except that rune strike doesn’t seem to be available as often as I expected, so I found myself using death coil more often as a runic power dump.  If this gets resolved, then single target threat should go up nicely.

Survivability is okay, but not great, as our armor has been significantly nerfed.  All tanks feel much more squishy now.  Of course, we DK’s now have a plethora of defensive cooldowns which is nice.
The worst part, by far, about blood DK tanking is being rune locked.  This is enough that I might not be tanking with my DK in cataclysm.

After the patch and my addons were stable, one of my guildies asked me to tank a “quick” 5 man for him.  In what I can only describe as an RNG curse, we get heroic halls of reflection.
Gauntlet fights as a DK tank (and possibly as any tank with the overall nerfs to all tanks) are the most frustrating gameplay I’ve ever encountered.

How the fuck does blizzard expect me to tank a gauntlet fight where mobs come in successive waves several seconds apart?  Adds were on the healer and there wasn’t a damn thing I could do about it.  Both taunts were on CD, runes were locked, RP is empty, empower rune weapon is on CD. WTF blizz!?  If there are gauntlet fights in cataclysm like this, I will curse blizzard to the day I die.

Unholy DPS:  Dark transformation is cool.  DPS’ing in unholy presence feels different, but I like the faster rune generation.  Unholy DPS seems to have been buffed in 4.0.  I was able to pull 5k on the boss dummy with no buffs, in tier 9, crap trinkets, and still figuring out the priority situation.  The new default power auras are…interesting.  I would like the ability to turn certain ones on or off, or even turn them all off, as I’m already used to building my own with the power auras addon.

I am going to try to save my general opinions on each role and overall mechanics for another post after I’ve had time to hash out my feelings on the individual classes I play the most.