I have had the same issue with my DK. I want to get him geared and so considered running him as part of another team as dps, as said above though, his dps is really poor and so it felt more like I was 4 manning most dungeons.
Ultimately I want to tank with him but he is only at 21k health even though his defense is at 545 and so makes it hard to do much tanking with him.
Hopefullly someone will have something good for dps in this thread :P