Wilson's issue with Kessel was leaving the zone too early or not coming back at all. Wilson made a point to highlight how much Kessel's defensive game improved this season. The other issue with Kessel that was pointed out is that he doesn't win battles to clear the puck or skates by the defender. Carlyle has mentioned that he wants to see more stops and starts from Kessel, rather than circles.
These are indeed issues with his defensive game but they are relatively minor ones. The puck doesn't always go to his side, and every Leaf winger has had problems clearing the zone this year. There isn't enough puck support...but anyways this isn't the place to discuss team issues. Kessel can be taught to play defense. He's still young and still growing as a player.
Regarding Kessel's physical game, there could be any number of reasons why he doesn't engage in physical activity often. One thought I've had on the issue (and I've mentioned it before) is that Kessel wants to stay healthy. Remember, he had major off season shoulder surgery 3 years ago. It may not be a nagging issue but it could be something in the back of his mind. Or maybe he's just a wuss? I don't know. Personally, I don't think that's the case, and frankly, I don't care either way. It's a minor issue as far as I'm concerned.