Kesler was a solid but not special defensive player who was perennially in the Selke conversation. Now Barkov is.
Barkov's goals against impacts are negative, meaning the Panthers give up more goals per 60 minutes with him on than with him off.
And as you can see here, the prime scoring areas are not a hard place to be when he's on the ice. His competition is nothing to write home about and his deployment is nothing to write home about.
He's not bad defensively by any stretch, but he's not special.
If anything, his offense is underrated. The Panthers take a crazy amount of shots and score (relatively) a crazy amount of goals when he's on the ice.