Yeah Malygos is the hardest encounter in Lich king, besides Satharion with 1 or more Drakes up. Malygos is a really fun fight though, I really enjoyed it. And Hero of the Frozen Wastes is a fucking awesome title.
On another note: Tanking with my death knight (a blood elf called Gattsu, go figure

) is fun as hell. Not only that I am a freaking AOE aggro machine, but tanking with random groups is so much more fun, and relaxing than healing with them.
As a tank I AM THE FREAKING GROUP. The group goes where I go, they kill the mobs I tank, when I eat bufffood, they wait, etc. And healers like me too

Before a pull I take a quick look at the groups, especially the healers, mana, and if it is way up I pull. If I get spiky damage I burn my cooldowns (which are nice on a 1 min cooldown only, not 4-5 mins like warriors shield wall or palabubbles).
When I healed with my priest in random groups, if the tank is bad it is always a disaster and really stressful. I have no mana, so I drink, tank pulls, group wipes, I laugh and explain to them that no mana = no heals. They whine.
Also, with all the tanking changes in Lich King it is so easy to hold AOE aggro. Healing still is stressful, you have to be there watching the healthbars every freaking second, otherwise there may be a wipe because of you. With tanking I make the tempo of the group. Also I can pretty much go afk after I've been through my rotation once and then just autoattack because I hold that much aggro with just a few skills (well at least on trashmobs, bosses are another thing of course).
Anyway I love the DK. Still raiding Naxx 25, Malygos and Satharion (tried once with 1 drake up, man it's freaking hard) with my Priestess as shadow. Doing Arena as a Holy Priest, but I am really thinking about making my DK my main.