How do people fail as DKs?

I mean, they are so easy to dps...

I'll start out by telling a faildk story from the other day.

My husband and I were on our shaman (at lvl 76) in a random instance. I was dps, he was healer, a warrior tank, a 77 hunter dps, and a 75 faildk dps. I was hitting 1800 dps easy, hunter was hitting 800 (not very good either), and the dk was hitting, are you ready for it...400dps... 400 DPS!!! *sighs* He never used icy touch or plague strike, did a couple death and decays, but only a couple, and pretty much white swung the entire instance. On one of the bosses a leather healing chestpeice dropped, and as shaman, my husband could not need, even though it was better. So he asked if he could have it from whoever got it. Well, the dk got it and refused to give it too him, saying if he needed it, he should have needed. No matter how much everyone in the party tried to explain that a shaman who wears mail can not need on anything but mail. *sigh again*

So, My husband started giving the dk a hard time about his dps, posting recount after fights where my husband (the healer) out dps'd him by 500+ dps...We got to a boss that does a shatter and he told me to go stand by him to kill him, and I said sure :P This dk was being a complete ass and figured we'd make his repair bill more than what he would sell the leather chestpeice for (yes, we are vindictive). So, hubby kept me healed, dk got shattered to death by me (made me feel better) So, we get to the last boss, and we told him to just give my hubby the leather chestpeice and we would stop hasseling him...he voted to kick my hubby out of the party and it passed (the other party members said "crap I thought that was to kick the dk") So, I said, "too bad, because I'm with him" and left the party with a warrior, and two faildps.

So, we jumped into another random, got the same instance and finished it in a 4th the time it took us to get to the last boss with the failgroup...And the dk that was in this new party...used pestilence! I was like "OMG the dk used diseases!" and my husband proclaimed to the group "Thank you so much, you just made my wife's night by using pestilence"

So...I made a dk on my hubby's account to run one of my random alts through some instances. How is it that a lvl 58 dk is doing 400 dps...the same as a lvl 75? It isn't that hard...icy touch/plague strike/blood strike/blood strike...