I don't care if he hits 50 home runs, if he hits .232 again and steals 10 bases I don't like his current draft position. Since both of those were career lows you are either in the school that believes he is fading due to age, or you think he will improve upon those numbers and keep his 35-40 HR power.
I'm in the second school of thought, and I'm expecting numbers around .250-95-35-100-10, but looking at who went after him in that draft (Cargo, Hamilton, Upton, A Jones) I don't think Granderson was great pick there. The next four OF's here could realistically put up those numbers and have a better BA. Granderson gets points for durability over CarGo and Hamilton, and Upton isn't very consistent, but at least these guys won't hurt you at all in BA and pitch in double digit SBs.
BA is as predictable as any stat. As ender said player's typically go up/down from a static point and last year Granderson was clearly down from his. As a .260 career hitter, I'd say his appetite for the longball has pushed his expected AVG down a bit. He had his highest K rate of his career last year, ran less and got dumped to the 6 hole in the lineup. This isn't someone I'm reaching for or expecting big improvement from, and to justify a pick inside the top 20 OA, he'd have to improve on last year's numbers or you will end up with a three category player early in round 2.