Karlsson is not the only hockey player who is finding out that a one-dimensional offensive game just doesn't cut it in the NHL. Everyone loves goalscorers. Goalscorers are worshipped at every level. There is no glory at all in defensive genius. How do I know that? Because the salaries for defensive players are limited.
The thing that NHL players fail to understand is that defensive ability is not a skill. It is simply a willingness to do the work. Anyone with offensive skills has the ability to play defence. What's missing is the desire to expend their energy on such a "trivial thing" as defence.
Obviously, if a coach decides that defence is not trivial, then it is not trivial. In Jason Spezza's case, no one except Team Canada coaches and Jacques Martin ever insisted that he needed to play defence if he wanted to play. This year he is becoming a complete player because of this insistence, and it's about time.
A dman who is all out offence needs to be filling the net every night for coaches to overlook any defensive shortcomings... otherwise they end up like Brendan Bell... playing in the KHL even though they are loaded with offensive skill.
Any team that allows several offensively talented players to just play offence ends up with no team... sort of like what you have in New Jersey with the arrival of Kovalchuk. How can a lowly coach even begin to insist that their gazillion dollar Kovalchuk play defence or he won't play???