Tried main tanking Drak'Tharon Heroic with my 80 DK. I had 25,000 life (in frost presence) and 502 defense. I'm wearing all defense gear blues from AH and Dungeon Drops. I had 14% dodge and 13% parry.
It went 'ok' but I was always having to pop icebound fortitude, unbreakable armor, and lichborne. On the Trex bos he was hitting me so hard we eventually wiped. It just seemed too much for the healer to heal through.
Is there something fundamentally flawed with DK's that prevent them from tanking as efficiently as warriors/druids?