Everything counters Frost DK's in 1v1's. Even Warriors do. Frost DK's and Arms Warriors both horrible at 1v1 and they're basically only good at destroying bad players who have no idea how to manage their defensive abilities or kite.
And Tyrannica, DK's are not hard to master. I'm assuming you're talking about Unholy. Unholy seems confusing at first, but once you have it down its probably easier to play than Frost.