Compared to say, a paladin, a DK does take more damage being that their mitigation works differently. Basically, you have to click it for it to work.
From a healers PoV, in 4.0.1 their health will yo-yo, it's something you have to keep on top of and I do think that DK's have had a bit more of a raw deal than those shield bearers. Not healed a big-fat-bear since the patch hit, would be interesting to compare.