While Sam's list is sweetly inclusive, generic and even more or less true if you're considering players to be developed most Pro teams don't take on many developmental players unless they already see quite a lot in that player to begin with.
I would add to Sam's list, experience, because unless you are Ollie jr. it frequently is a critical factor in being successful at the top levels. Experience should make for smarter players and should indicate some measure of mental toughness that if you lack none of the rest matters all that much.
Beyond that show me speed, aggression and good decision-making in the game on environment and you'll get my attention. Not necessarily in that order.