Kessel at the time was worth more than a typical "5th overall". As stated, he was a proven commodity of 30+ goals with his best years ahead of him. However, his value was marginalized by the fact he was up for a new contract and wanted top $'s. Giving up picks for an elite young player is one thing. But when you have to pay that player $5.5M per season, AND give up a couple of 1st rounders and a 2nd, that is the tough part. But the Leafs had cap room, and it seemed like good value. I still think Kessel is a great sniper to have, presuming you can get him the right linemates (like he had in Boston).
The real miscalculation by Burke of course, was he never anticipated he would be giving up a couple of lottery picks. There is no way he would do that deal if he knew that. Frankly, if Burke could swap Kessel for Seguin right now, he'd do it (although he'll never admit that). But that was the risk he took and he knew that was part of the gamble I'm sure. The Bruins were up against the cap wall and this was a tremendous deal for Chiarelli to make, even if they had to take a small step back last year, they will be many leaps ahead a few years from now.