Johnny has every single physical attribute necessary to be a very good NFL QB. He's not a completely reckless runner and tends to be very good about protecting himself on the field. He reminds me of a more talented Russell Wilson. Wilson knew how to minimize his size disadvantage, as does Manziel. His arm isn't a cannon, but it's definitely good enough.
That said, Johnny does not have the leadership or maturity necessary to be an NFL quarterback. Whether it's fair or not, if you're a starting QB in the NFL, anything you do is going to be put under a microscope and picked apart. I just don't see Johnny thriving in an environment like that, or coaches or his teammates liking Johnny Football being Johnny Football.
Even as a Bama fan, I'm rooting for Johnny to succeed. But unless he matures into a much better leader, his ceiling is right around Cam Newton.
All this said, if he goes to the Eagles in the draft, watch out.