It's age, arm length and lateral mobility.

Given that he's performed very well at the college level, I expect him to be solid in the NFL, but those traits worry teams enough that I think he'll be a 3rd rounder. It's not smart draft strategy but teams hunt the homeruns (as opposed to the solid starters) in early rounds.

See someone like Ziggy Ansah (who did work out) or Margus Hunt (who didn't really)