Is Reilly a 1D? Karlsson? Burns? All have major shortcomings.
What exactly is the 1D role?
I would say no, yes, no. Everyone's got their own definition of what a 1D is, it's easier to decide if a guy is a 1D than define what a 1D is. The more well rounded guys like Hedman, Pietrangelo, Doughty are obvious but I think the more one-dimensional like Karlsson can be too. I think there's quite a few teams that lack a true 1D, my own team, the Flyers, included.
I feel like a 1D is a guy you can trust in all situations to be able to defend reasonably well and turn any situation to your team's favor more often than not. Trouba isn't a guy that I would always have that confidence in. Burns and Reilly are guys that I would, again, not feel confident in in all situations. They're borderline but they can be taken advantage of by certain players or teams. Karlsson isn't the greatest defender but his unique offensive talent is such that he can create advantage out of nothing in ways other guys can't while still being able to defend reasonably well.
It's a hard quality to pin down but something you know when you see it. I can understand people who want to define it by "is he a top 31 defenseman in the league? Yes? Then he's a 1D." I just think asking if someone is a top 31 D or if they're a 1D are different questions.