I am a season ticket holder who has seen him live 16 times this season and on TV several more. I concur that he has greatly improved from the beginning of the year. He has adjusted to the speed and the size of the other players, but especially he has learned to trust his linemates. His no longer tries to do everything himself.
Here are some additional considerations:
Kessel plays on the second line with another freshman (Blake Wheeler), and a sophomore who was not a regular last year (Ben Gordon). If the team concentrated their top three forwards all on the same line, his numbers would be even better.
As mentioned earlier, he plays regularly on the penalty kill. That alone should indicate that his defense is not quite as suspect as his reputation on these boards.
His plus minus (another indication of defensive proclivity) is +11, which is only four off the top +/- on the team.
Areas where he can improve?
1) shooting accuracy - He averages 5 shots a game, which is outstanding, but is converting only 9% of his shots.
2) strength - He doesn't seem like he's quite as big as he measures. He's not a weakling, but a little extra muscle will help him in many ways.
3) face offs - he wins 51% of his faceoffs. Not bad, but clearly there is room for improvement.