My main toon, before 5 boxing, is an 80DK. I've run DW Frost DPS, Unholy 2H DPS, Unholy PvP, and Frost Tanking (Both DW and 2H). So, please hear me when I say:

Run. Run fast and far away from DK Tanks! They are okay to solo, but still maybe the hardest tank to solo well. I can't even fathom trying to MBox with them as a Tank personally. The biggest is reason is they take damage in spikes (whereas a druid is a bit more smooth in taking damage IMO). You'll spend WAY too much time focusing on keeping a DK healed than other tasks.

As others have mentioned the Pally is so easy to tank with. You really CAN just macro spam 1 button and do just fine.

Lastly, I agree that healing is trickier/harder than tanking. With tanking, once you get the mechanics down, they are pretty much the same from there on out; now and then you'll have to learn a fight, but not a lot changes. It's about situational awareness, and reaction if things go awry (aggro moves, etc). Healing is situational awareness and prediction. If you are healing people based on "reaction", likely you're wiping groups, especially if you have a DK tanking

My 2 cents, your mileage may vary.