My DK tank (team is now 75-77) has leveled as hybrid Frost/Blood (for the Icy Touch haste buff + blood 10% AP buff). I've done instances almost exclusively from 72 on, just to collect blue gear and fill in holes in questing (because I can't be bothered to do collection quests!).

His gear is quest rewards and the occasional +Def or "tank" item. I'm not worried about defense caps or other tank gear specialization until he hits L80 and is ready to start doing heroics. You don't need a great "tank set" from 60-80.

I plan on using full Savage Saronite set at L78 for all my team, including the DK. At 80, I will run him through instances on his own for a week or so until he has the tank gear necessary to have an easier time at heroic farming.

So... from 60-80, don't sweat it if you're not in the perfect tank gear or spec... You'll probably be just fine.

(Just don't forget Frost Presence!) :P