Poor death knights are at a serious disadvantage when it comes to speed clearing low level content. There are only so many runes to go around. Logic dictates how hard can it be if you can solo the instance, but the reality is a little different.
First off by low level I mean like 60-70 content; easy, but you can still get hurt. Once you get to the DPS can one shot mobs level of easy tanking is irrelevant.
Death knights have a rhythm to them, patterns of abilities which anticipate which runes will become available. Speeding up pulls breaks that rhythm. Light bulb; I'll pull more at a time. Kills two birds with one stone, it's less boring and I get to keep my rotation more consistent with level appropriate content.
Oh, bad. Everyone pulled off me, guess I should of let people know what I was doing. I forgot when a mob moves that means a tank has it. Things worked out better once I shared what I was doing.
All in all I think I will stick with the bear for tanking low level content. A full rage bar is easy to maintain with all those dodges and there are no cooldowns limiting the spamming of swipe, a much more organic do what you want when you want style of tanking.
As for soloing old content, feral druids rule. I've been going back to old instances in Azeroth for the achievements lately. Kind of fun with the exception of Wailing Caverns so far. Two more to go then I will do the same in Outlands. I'm interested to find out if I can handle a heroic instance on my own, but am optimistic.