I've been levelling a (blood spec) DK tank. It's interesting to see the differences between a blood DK and a warrior, although I understand that frost and unholy specs play a little differently.
Firstly, I am amazed at how overpowered the DK is in solo content. I can take on 6 or 7 mobs of the same level or greater and come out with full health. The self healing is really something special. I can easily solo elites of higher level (orange to me). Possibly one of the reasons that there are so many bad max-level DKs is that the DK is so very easy to level - run in, drop a DnD, spread your diseases (optional), death strike when possible to restore health.
Disease management is annoying when tanking. The diseases don't seem to last that long or be autorefreshed (or maybe I haven't picked up the talent or ability that refreshes them yet). Keeping multiple mobs diseased via pestilence is something that often gets lost during chaotic fights.
It's also annoying that you need to press four buttons to do decent AoE damage - frost strike, plague strike, pestilence, blood boil. That's 6 seconds for impatient dps to pull aggro from you. DnD is great, but it's on a long cooldown so is not always available for every pull or if adds arrive late in the fight. Warriors may complain that DnD is better than thunderclap, and indeed it is better in theory, but thunderclap is available every six seconds and hits hard enough to get the mobs onto you when you use it. Frost DKs get howling blast, so I guess this is something unique to blood.
I wonder how the rune changes will alter rotations for DKs in Cata. I haven't read anything recently but I recall the intent was to slow down rune recharge times by only allowing one rune of each type to recharge at any one time. Certainly DnD looks absurdly expensive for three runes if the rate of recharge is essentially halved - it will mean I think that you can't use any more rune-based attacks for 10 seconds afterwards since it would put a frost, blood and unholy rune on cooldown. Perhaps rune strike going onto the GCD will help DK tanks have a button to hit in those circumstances. In addition if your diseases last 15 seconds and you are only going to get a frost rune every 10 seconds, most of your frost runes will need to be spent on keeping Frost Fever up I think. This means death strike could be a bit of a luxury.
DPS is insane as usual - I am in tanking gear, with no dps talents outside the cookie cutter, only the heirloom shoulders and I am pretty much always top of the meters, and even single target it's possible to pull 500 dps or so. Nerf the buggers!
DKs have some amazing abilities - strangulate and chains of ice, for example - which seem to be lacking in other tanks and I think their "toolkit" is probably the best of any class other than the warrior. The two-resource system is just a bit too unwieldy to be comfortable - I see many complaints about the resources not being there when you need them for interrupts and for defensive cooldowns, and personally I find the requirement to have a blood rune available for rune tap, for example, to be potentially fatal.
My overall reaction is that there is just something a bit too clever about the DK class for me. I'm enjoying the levelling curve, but I don't think that my warrior is in danger of being replaced any time soon.