Having spent many nights and afternoons at the Hershey Centre in Mississauga this season, I can confidently say that Sean Day is a highly talented hockey player. My viewings this season ranged from noticeably bad to best player on the ice.
There were games where the elite skating and puck rushing ability were on full display. But there were other games where Day was a turnover machine who skating himself directly into traffic. The former looks like a top-30 pick, a potential top-four defender in the NHL; the latter, perhaps a late round flyer at best.
While the physical tools are immediately striking, the decision-making and consistency are what hold him back. If a team feels that the decision-making can be fixed, then he would be a very worthwhile gamble as early as the second round. If not, perhaps he's not even worth a draft pick.
Tossing away the off-ice and work ethic stuff (as I have no way knowing about it), I think Day is a worthwhile gamble for the Montreal Canadiens. Certainly not in the first or second, but in the third or later, if he lasts that long.