Yeah, last year's Karlsson was that mix of good and bad - exceptionally good offensively, generally decent but occasionally dominated in the defensive zone. I didn't like him winning the Norris just because he had gaudy point totals. I thought both Chara and Weber had more impact on the game when they were on the ice.
But you haven't seen Karlsson much this year if you still think he's a one-dimensional player. The only reasons he doesn't play more on the PK are his overall ice time (over 27 min./game) and the fact that the Sens have some capable PK D who can take that role on for him. Karlsson now uses his skating ability much more effectively defending players in open ice, and his quickness compensates for the fact he doesn't clear guys out down low. He just keeps the puck from getting to them. He has been much more impactful this year - more than Chara who is, it seems, having a slow start.