Best player I've ever come across in Crucible has a 2.57 k/d. I know because I had to go look it up after the absolute thrashing he put on me and the rest of the team in several Control matches. I learned that he was a top 1% and he certainly played like it. I don't think that k/d is even representative of what he does regularly these days. Anyways, point being is that you'll know a great player when you encounter one. K/D will only tell some of the story but they most likely will be 2.0<.