All DK's have been tanking as DK's since November 13th, give or take playing in beta and the time it took to level to a point where there was stuff to tank. None of them have experienced MTing MC, BWL, AQ40, Naxx40, SSC, TK, BT, Hyjal, Sunwell....as a Death Knight. Sure, things change over time and warriors today are much different than a warrior stepping into Vael for the first time but there is something to be said for all that experience.
A fair number of DKs are rerolls from classes that have never tanked before. Rolling /need against a dude who had been a mediocre mage 3 months ago is a little disconcerting. Because you can, doesn't mean you should. Many tanks are fixtures in their guild and have been a rock for them and a known quantity. Damn few people can honestly say they have truly earned the MT spot in a month of live server play as a new character. There are rare cases of a tank rerolling to a DK I suppose but it has much less to do with the class mechanics than it does with experience, attitude and resume.
your hat may be nice, but I have the little white tank top that says Legendary right across my boobs. I win. (or more correctly, H wins)