Heroic mode instances have been released on the beta and I managed to level up another DK in time and quickly get together some form of a tanking gear set. My gear was far below what the heroic instances are actually (supposed to be) tuned for. Below follows a small summary of my experience the last couple of days on the beta.
The tanks are far and few on the beta, and DKs are arguably the strongest tanks together with Druids at the moment, though gimmicks as always gib us quite hard (more on that later).
I never had much of an issue finding groups. Before I set foot in most of the heroics I hadn't seen to much of all the normal dungeons, so there was a lot of learning through trial and error and I eventually adapted my playstyle, gear, enchants, and spec.
Gear and stats
Avoidance is gold worth, that much I can say without hesitating. Of course you need a reasonable ammount of health, but that will mostly come by default through gear. The way we're designed with Rune Strike at the moment also promotes stacking avoidance to obscene (28%!!) levels. I didn't really bother stacking Mastery too much, even though it scales up quite quickly.
I've found the Blood Shield to, in several situations, be hard to get the full effect out of. It requires the tank's involvement to activate, and even if you're a great player and manage to get the full effect out of it (no, I didn't manage perfectly, took me some time to get used to its mechanic), there will be situations where you just won't be able to, due to various restrictions of the encounters you're up against.
Avoidance always grants its full effect without the players involvement, though there's always the RNG argument (which for the most part is invalid when we can't be 2-shot anymore).
The actual loot that drops in the instances is quite good as it is, and reforging makes pretty much ever item great. I'm even using a few reforged DPS parts. Still haven't seen more than one tanking trinket though.
Combat
Over the course of the heroics I did, I definitely started to appreciate our Spec bonus Blood Rites (formerly Death Rune Mastery). The flexibility it provides probably makes us the strongest AoE tank in many situations. It's even perfectly fine without the application of diseases through Pestilence. It actually looks like most classes have had their AoE capabilities increased since Wrath.
On the topic of AoE, the loss of the glyph of Pestilence annoyed me at first, but with a lot of CC going on, it was often better not to have it. Running the mobs a few yards more together before using was really a non-issue.
Our self-healing may feel nerfed in some ways, but it's definitely still quite powerful, partly because of the damage to health ratios and healers having limited mana. Blood Worms are simply awesome.
Since we're quite resource-bound under the new rune system, most boss pulls I initiated with Dancing Rune Weapon and dished out as much threat I could in order to be able to "slack off" further into the fight and handle adds, self-healing, CD-usage and such much more smoothly. Dancing Rune Weapon is powerful, and feels nice when you proc a few more Rune Strikes, but I would like for it to be able to copy my Outbreak (perhaps I'm asking for too much).
The only negative feedback I have on DRW would be that it's a "no-skill" ability. You can't use it wisely, it just makes you 50% more powerful.
Bosses and Trash
First and foremost, a lot of this felt only roughly tuned. It's the first pass on the heroic dungeons and I expect some to be tuned up and others down. We spent half a day in Stonecore heroic, and cleared Vortex Pinnacle without any real issues, for instance. Secondly, gear is hard to come by and most are really under-geared for heroics at this point.
Crowd control is most certainly back, and I love it. Pretty much every class has some form of control now, and I'd even count DKs into that category. Locking out a caster with Mind Freeze, Strangulate and Hungering Cold/Gnaw is also more fun than most CCs! The Unholy pet can even tank an add or two.
From a tanking perspective, several bosses are actually just as engaging as raid bosses are, some even more engaging. Tank movement, add control and cooldown management is all there, though depending on how the heroics end up getting tuned this could possibly be negated. As of now there are several boss abilities that are definitely deadly and some even one-shot if you don't handle them correctly. Great change to demand more of the tank as opposed to bosses a la Blood-Queen.
This post has turned into a wall of text and it's getting pretty late so I'll wrap it up here by saying I'm quite impressed with what Blizzard has done so far. I don't even dare imagining what the raids will look like.
Here's some of the runs I did (props to TotalBiscuit)
Vortex Pinnacle Heroic
Stonecore Heroic (part 1)
Stonecore Heroic (part 2)

