His defensive game is basically the good Duncan Keith. But yea, prepare yourself for Doughty/Kings fans calling him a 4th forward or a defensive liability. They couldn't be further from the truth. His stick is elite and when you combine that with his skating he's one of the best players in the league at getting the puck back. He is also incredible with the puck in his own end, which to me is just as much of a defensive trait as it is offensive.
Defence is all about getting the puck and moving it out from your own zone. Karlsson is one of, if not the best at both. And yea, he isn't perfect, sometimes he can lose his man on the cycle, he's no shea weber in front of his own net, and his pivoting to the right has been **** ever since Cooke cut him.
But he can make defensive plays that no other D can make. The way he can turn defence into offence is on another level. He's probably the best all around player in the NHL.