You also have to remember that most level 80 dks are, in theory, level 25. They're new to the class, and have only played 25 levels, so there's still a lot of learning to do. I'm starting to really get the hang of using my DK for a tank for my 4 shaman. You possibly just haven't grouped with people that understand tanking. When I single boxed, one of my mains was a feral druid tank, so I guess I already had the basic gist down of tanking.