I don't have an issue with the way Karlsson gives guys space because he's quick enough to take it away before the forwards even know it. Also, he's not going to really be able to win the physical battles that Kronwall can win either simply because of his size.
Karlsson is best described as a sneeky D man without the puck, yah he makes his mistakes and sometimes gets caught out of position but he can also cause a turn over and before the other team knows they don't have the puck anymore Karlsson is at center ice.
All in all I like the way he plays the position based on how he generally plays the game. I don't expect a guy like Cowen to turn and go and I don't expect Karlsson to take away time and space and get physical. Lots of different ways to play that position and it drives me nuts how some people have a cookie cutter idea of how a D man should play.
Defense is a position that 2 players on a team play at any given time while on the ice, it isn't a description of what they should be doing.